[DOCS] Add data streams to field caps API docs (#59326)

docs/reference/search/field-caps.asciidoc

@@ -2,6 +2,8 @@
 === Field Capabilities API
 
 Allows you to retrieve the capabilities of fields among multiple indices.
+For data streams, the API returns field capabilities among the stream's backing
+indices.
 
 [source,console]
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@@ -16,9 +18,9 @@ GET /_field_caps?fields=rating
 
 `POST /_field_caps`
 
-`GET /<index>/_field_caps`
+`GET /<target>/_field_caps`
 
-`POST /<index>/_field_caps`
+`POST /<target>/_field_caps`
 
 
 [[search-field-caps-api-desc]]
@@ -32,7 +34,13 @@ fields among multiple indices.
 [[search-field-caps-api-path-params]]
 ==== {api-path-parms-title}
 
-include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=index]
+`<target>`::
+(Optional, string)
+Comma-separated list of data streams, indices, and index aliases used to limit
+the request. Wildcard expressions (`*`) are supported.
++
+To target all data streams and indices in a cluster, omit this parameter or use
+`_all` or `*`.
 
 
 [[search-field-caps-api-query-params]]
@@ -104,7 +112,7 @@ field types are all described as the `keyword` family type.
 ==== {api-examples-title}
 
 
-The request can be restricted to specific indices:
+The request can be restricted to specific data streams and indices:
 
 [source,console]
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