[Docs] Retrieving metadata using the fields option (#79174)

Adding a small section to the field retrieval article about which metadata
fields now can be retrieved via the `fields` option.

docs/reference/search/search-your-data/retrieve-selected-fields.asciidoc

@@ -376,6 +376,17 @@ won't be included in the response because `include_unmapped` isn't set to
 // TESTRESPONSE[s/"max_score" : 1.0/"max_score" : $body.hits.max_score/]
 // TESTRESPONSE[s/"_score" : 1.0/"_score" : $body.hits.hits.0._score/]
 
+[discrete]
+[[retrieve-metadata-fields]]
+==== Retrieving metadata fields
+By default, document metadata fields like `_id` or `_index` are not returned
+when the requested `fields` option uses wildcard patterns like `*`. However,
+when explicitly requested using the fields name, the `_id`, `_routing`,
+`_ignored`, `_index` and `_version` metadata fields can be retrieved.
+In addition, when you define an <<field-alias, alias field>> with a path to one
+of the above metadata fields, this field will also be retrievable using either
+the alias name or via wildcard patterns that match the alias.
+
 [discrete]
 [[Ignored-field values]]
 ==== Ignored field values