[DOCS] Reformat update API reference. (elastic#45423)

* [DOCS] Reformat update API reference.

Author: debadair

docs/reference/docs/index_.asciidoc

@@ -34,7 +34,7 @@ using a PUT request. Omit to automatically generate an ID when using a
 POST request.
 
 
-[[docs--api-query-params]]
+[[docs-index-api-query-params]]
 ==== {api-query-parms-title}
 
 include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-seq-no]

docs/reference/docs/update.asciidoc

@@ -1,19 +1,86 @@
 [[docs-update]]
 === Update API
+++++
+<titleabbrev>Update</titleabbrev>
+++++
 
-The update API allows to update a document based on a script provided.
-The operation gets the document (collocated with the shard) from the
-index, runs the script (with optional script language and parameters),
-and indexes back the result (also allows to delete, or ignore the
-operation). It uses versioning to make sure no updates have happened
-during the "get" and "reindex".
+Updates a document using the specified script.
 
-Note, this operation still means full reindex of the document, it just
-removes some network roundtrips and reduces chances of version conflicts
-between the get and the index. The `_source` field needs to be enabled
-for this feature to work.
+[[docs-update-api-request]]
+==== {api-request-title}
 
-For example, let's index a simple doc:
+`POST /<index/_update/<_id>`
+
+[[update-api-desc]]
+==== {api-description-title}
+
+Enables you script document updates. The script can update, delete, or skip
+modifying the document. The update API also supports passing a partial document,
+which is merged into the existing document. To fully replace an existing
+document, use the <<docs-index_,`index` API>>.
+
+This operation:
+
+. Gets the document (collocated with the shard) from the index.
+. Runs the specified script.
+. Indexes the result.
+
+The document must still be reindexed, but using `update` removes some network
+roundtrips and reduces chances of version conflicts between the GET and the
+index operation.
+
+The `_source` field must be enabled to use `update`. In addition to `_source`,
+you can access the following variables through the `ctx` map: `_index`,
+`_type`, `_id`, `_version`, `_routing`, and `_now` (the current timestamp).
+
+[[docs-update-api-path-params]]
+==== {api-path-parms-title}
+
+`<index>`::
+(Required, string) Name of the target index. By default, the index is created
+automatically if it doesn't exist. For more information, see <<index-creation>>.
+
+`<_id>`::
+(Required, string) Unique identifier for the document to be updated.
+
+[[docs-update-api-query-params]]
+==== {api-query-parms-title}
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-seq-no]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-primary-term]
+
+`lang`::
+(Optional, string) The script language. Default: `painless`.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-refresh]
+
+`retry_on_conflict`::
+(Optional, integer) Specify how many times should the operation be retried when
+ a conflict occurs. Default: 0.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-refresh]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-routing]
+
+`_source`::
+(Optional, list) Set to `false` to disable source retrieval (default: `true`).
+You can also specify a comma-separated list of the fields you want to retrieve.
+
+`_source_excludes`::
+(Optional, list) Specify the source fields you want to exclude.
+
+`_source_includes`::
+(Optional, list) Specify the source fields you want to retrieve.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=timeoutparms]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=doc-wait-for-active-shards]
+
+[[update-api-example]]
+==== {api-examples-title}
+
+First, let's index a simple doc:
 
 [source,js]
 --------------------------------------------------
@@ -25,10 +92,8 @@ PUT test/_doc/1
 --------------------------------------------------
 // CONSOLE
 
-[float]
-==== Scripted updates
-
-Now, we can execute a script that would increment the counter:
+To increment the counter, you can submit an update request with the
+following script:
 
 [source,js]
 --------------------------------------------------
@@ -46,8 +111,8 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-We can add a tag to the list of tags (if the tag exists, it
- still gets added, since this is a list):
+Similarly, you could use and update script to add a tag to the list of tags
+(this is just a list, so the tag is added even it exists):
 
 [source,js]
 --------------------------------------------------
@@ -65,11 +130,11 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-We can remove a tag from the list of tags. Note that the Painless function to
-`remove` a tag takes as its parameter the array index of the element you wish
-to remove, so you need a bit more logic to locate it while avoiding a runtime
-error. Note that if the tag was present more than once in the list, this will
-remove only one occurrence of it:
+You could also remove a tag from the list of tags. The Painless
+function to `remove` a tag takes the array index of the element
+you want to remove. To avoid a possible runtime error, you first need to
+make sure the tag exists. If the list contains duplicates of the tag, this
+script just removes one occurrence.
 
 [source,js]
 --------------------------------------------------
@@ -87,11 +152,8 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-In addition to `_source`, the following variables are available through
-the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
-and `_now` (the current timestamp).
-
-We can also add a new field to the document:
+You can also add and remove fields from a document. For example, this script
+adds the field `new_field`:
 
 [source,js]
 --------------------------------------------------
@@ -103,7 +165,7 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-Or remove a field from the document:
+Conversely, this script removes the field `new_field`:
 
 [source,js]
 --------------------------------------------------
@@ -115,9 +177,9 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-And, we can even change the operation that is executed.  This example deletes
-the doc if the `tags` field contains `green`, otherwise it does nothing
-(`noop`):
+Instead of updating the document, you can also change the operation that is
+executed from within the script.  For example, this request deletes the doc if
+the `tags` field contains `green`, otherwise it does nothing (`noop`):
 
 [source,js]
 --------------------------------------------------
@@ -136,13 +198,8 @@ POST test/_update/1
 // TEST[continued]
 
 [float]
-==== Updates with a partial document
+===== Update part of a document
 
-The update API also supports passing a partial document,
-which will be merged into the existing document (simple recursive merge,
-inner merging of objects, replacing core "keys/values" and arrays).
-To fully replace the existing document, the <<docs-index_,`index` API>> should
-be used instead.
 The following partial update adds a new field to the
 existing document:
 
@@ -158,14 +215,14 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-If both `doc` and `script` are specified, then `doc` is ignored. Best is
-to put your field pairs of the partial document in the script itself.
+If both `doc` and `script` are specified, then `doc` is ignored. If you
+specify a scripted update, include the fields you want to update in the script.
 
 [float]
-==== Detecting noop updates
+===== Detect noop updates
 
-If `doc` is specified its value is merged with the existing `_source`.
-By default updates that don't change anything detect that they don't change anything and return `"result": "noop"` like this:
+By default updates that don't change anything detect that they don't change
+anything and return `"result": "noop"`:
 
 [source,js]
 --------------------------------------------------
@@ -179,9 +236,8 @@ POST test/_update/1
 // CONSOLE
 // TEST[continued]
 
-If `name` was `new_name` before the request was sent then the entire update
-request is ignored. The `result` element in the response returns `noop` if
-the request was ignored.
+If the value of `name` is already `new_name`, the update
+request is ignored and the `result` element in the response returns `noop`:
 
 [source,js]
 --------------------------------------------------
@@ -200,7 +256,7 @@ the request was ignored.
 --------------------------------------------------
 // TESTRESPONSE
 
-You can disable this behavior by setting `"detect_noop": false` like this:
+You can disable this behavior by setting `"detect_noop": false`:
 
 [source,js]
 --------------------------------------------------
@@ -217,11 +273,11 @@ POST test/_update/1
 
 [[upserts]]
 [float]
-==== Upserts
+===== Upsert
 
 If the document does not already exist, the contents of the `upsert` element
-will be inserted as a new document.  If the document does exist, then the
-`script` will be executed instead:
+are inserted as a new document.  If the document exists, the
+`script` is executed:
 
 [source,js]
 --------------------------------------------------
@@ -244,11 +300,10 @@ POST test/_update/1
 
 [float]
 [[scripted_upsert]]
-===== `scripted_upsert`
+===== Scripted upsert
 
-If you would like your script to run regardless of whether the document exists
-or not -- i.e. the script handles initializing the document instead of the
-`upsert` element -- then set `scripted_upsert` to `true`:
+To run the script whether or not the document exists, set `scripted_upsert` to
+`true`:
 
 [source,js]
 --------------------------------------------------
@@ -274,10 +329,10 @@ POST sessions/_update/dh3sgudg8gsrgl
 
 [float]
 [[doc_as_upsert]]
-===== `doc_as_upsert`
+===== Doc as upsert
 
-Instead of sending a partial `doc` plus an `upsert` doc, setting
-`doc_as_upsert` to `true` will use the contents of `doc` as the `upsert`
+Instead of sending a partial `doc` plus an `upsert` doc, you can set
+`doc_as_upsert` to `true` to use the contents of `doc` as the `upsert`
 value:
 
 [source,js]
@@ -343,19 +398,3 @@ matches the one specified.
 
 [NOTE]
 .The update API does not support versioning other than internal
-=====================================================
-
-External (version types `external` and `external_gte`) or forced (version type `force`)
-versioning is not supported by the update API as it would result in Elasticsearch 
-version numbers being out of sync with the external system.  Use the
-<<docs-index_,`index` API>> instead.
-
-=====================================================
-
-`if_seq_no` and `if_primary_term`::
-
-Update operations can be made conditional and only be performed if the last
-modification to the document was assigned the sequence number and primary
-term specified by the `if_seq_no` and `if_primary_term` parameters. If a
-mismatch is detected, the operation will result in a `VersionConflictException`
-and a status code of 409. See <<optimistic-concurrency-control>> for more details.