api docs: Document changes in API topic encoding.

templates/zerver/api/changelog.md

@@ -258,6 +258,11 @@ below features are supported.
 
 * [`POST /messages`](/api/send-message): Added support for using user
   IDs and stream IDs for specifying the recipients of a message.
+* [`POST /messages`](/api/send-message), [`POST
+  /messages/{message_id}`](/api/update-message): Added support for
+  encoding topics using the `topic` parameter name.  The previous
+  `subject` parameter name was deprecated but is still supported for
+  backwards-compatibility.
 * [`POST /set-typing-status`](/api/set-typing-status): Added support for specifying the
   recipients with user IDs, deprecating the original API of specifying
   them using email addresses.

zerver/openapi/zulip.yaml

@@ -5008,6 +5008,9 @@ components:
         (`type="stream"`), ignored otherwise.
 
         Maximum length of 60 characters.
+
+        **Changes**: New in Zulip 2.0.  Previous Zulip releases encoded
+        this as `subject`, which is currently a deprecated alias.
       schema:
         type: string
       example: Castle