api-docs: Add changelog entry for user/stream ID narrow options.

Adds a 2.1 release changelog entry for adding support for user
and stream IDs in search/narrow options. Also, adds a Changes
note in the narrow parameter in the OpenAPI `get-messages`
endpoint definition.

Both link to the api documentation for constructing a narrow,
where the 2.1 release update is already mentioned.

Fixes #9474.

templates/zerver/api/changelog.md

@@ -1041,6 +1041,9 @@ No changes; feature level used for Zulip 3.0 release.
 
 ## Changes in Zulip 2.1
 
+* [`GET /messages`](/api/get-messages): Added support for
+  [search/narrow options](/api/construct-narrow) that use stream/user
+  IDs to specify a message's sender, its stream, and/or its recipient(s).
 * [`GET /users`](/api/get-users): Added `include_custom_profile_fields`
   to request custom profile field data.
 * [`GET /users/me`](/api/get-own-user): Added `avatar_url` field,

zerver/openapi/zulip.yaml

@@ -4859,6 +4859,10 @@ paths:
           description: |
             The narrow where you want to fetch the messages from. See how to
             [construct a narrow](/api/construct-narrow).
+
+            **Changes**: In Zulip 2.1, added support for using user/stream IDs
+            when constructing narrows for a message's sender, its stream and/or
+            its recipient(s).
           content:
             application/json:
               schema: