Add conversation_rating behavior documentation to Unstable spec

Apply the same versioning documentation to the Unstable API spec.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: markdennis22

descriptions/0/api.intercom.io.yaml

@@ -18230,8 +18230,10 @@ components:
       title: Conversation Rating
       type: object
       nullable: true
-      description: The Conversation Rating object which contains information on the
-        rating and/or remark added by a Contact and the Admin assigned to the conversation.
+      description: |-
+        The Conversation Rating object which contains information on the rating and/or remark added by a Contact and the Admin assigned to the conversation.
+
+        Note: Starting from version 2.15, this returns the latest completed conversation rating (one with an actual rating value). If no completed ratings exist, it falls back to the latest rating. In earlier versions, this returns the latest rating by ID regardless of completion status.
       properties:
         rating:
           type: integer