ingest: document index.default_pipeline (elastic#34500)

Author: jakelandis

docs/reference/index-modules.asciidoc

@@ -222,6 +222,13 @@ specific index module:
     The maximum length of regex that can be used in Regexp Query.
     Defaults to `1000`.
 
+  `index.default_pipeline`::
+
+    The default <<ingest,ingest node>> pipeline for this index. Index requests will fail
+    if the default pipeline is set and the pipeline does not exist. The default may be
+    overridden using the `pipeline` parameter. The special pipeline name `_none` indicates
+    no ingest pipeline should be run.
+
 [float]
 === Settings in other index modules
 

docs/reference/ingest.asciidoc

@@ -35,6 +35,9 @@ PUT my-index/_doc/my-id?pipeline=my_pipeline_id
 // CONSOLE
 // TEST[catch:bad_request]
 
+An index may also declare a <<dynamic-index-settings,default pipeline>> that will be used in the
+absence of the `pipeline` parameter.
+
 See <<ingest-apis,Ingest APIs>> for more information about creating, adding, and deleting pipelines.
 
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