Clarify the settings around limiting nested mappings. (#42686)

* Previously, we mentioned multiple times that each nested object was indexed as its own document. This is repetitive, and is also a bit confusing in the context of `index.mapping.nested_fields.limit`, as that applies to the number of distinct `nested` types in the mappings, not the number of nested objects. We now just describe the issue once at the beginning of the section, to illustrate why `nested` types can be expensive.
* Reference the ongoing example to clarify the meaning of the two settings.

Addresses #28363.

Author: jtibshirani

docs/reference/mapping.asciidoc

@@ -89,10 +89,7 @@ causing a mapping explosion:
     `2`, etc. The default is `20`.
 
 `index.mapping.nested_fields.limit`::
-    The maximum number of `nested` fields in an index, defaults to `50`.
-    Indexing 1 document with 100 nested fields actually indexes 101 documents
-    as each nested document is indexed as a separate hidden document.
-
+    The maximum number of distinct `nested` mappings in an index, defaults to `50`.
 
 [float]
 == Dynamic mapping

docs/reference/mapping/types/nested.asciidoc

@@ -195,10 +195,20 @@ phase.  Instead, highlighting needs to be performed via
 
 =============================================
 
-[[limit-number-nested-fields]]
-==== Limiting the number of `nested` fields
+[float]
+=== Limits on `nested` mappings and objects
 
-Indexing a document with 100 nested fields actually indexes 101 documents as each nested
-document is indexed as a separate document. To safeguard against ill-defined mappings
-the number of nested fields that can be defined per index has been limited to 50. See
-<<mapping-limit-settings>>.
+As described earlier, each nested object is indexed as a separate document under the hood.
+Continuing with the example above, if we indexed a single document containing 100 `user` objects,
+then 101 Lucene documents would be created -- one for the parent document, and one for each
+nested object. Because of the expense associated with `nested` mappings, Elasticsearch puts
+the following setting in place to guard against performance problems:
+
+`index.mapping.nested_fields.limit`::
+
+    The `nested` type should only be used in special cases, when arrays of objects need to be
+    queried independently of each other. To safeguard against poorly designed mappings, this setting
+    limits the number of unique `nested` types per index. In our example, the `user` mapping would
+    count as only 1 towards this limit. Defaults to 50.
+
+Additional background on this setting can be found in <<mapping-limit-settings>>.