add mention of default pipeline

Author: jakelandis

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.
 
 --