doc: add return value for DC unsubscribe

Author: simon-id

doc/api/diagnostics_channel.md

@@ -267,10 +267,11 @@ added:
 changes:
   - version: REPLACEME
     pr-url: https://github.com/nodejs/node/pull/40433
-    description: Added to channels without subscribers.
+    description: Added return value. Added to channels without subscribers.
 -->
 
 * `onMessage` {Function} The previous subscribed handler to remove
+* Returns: {boolean} `true` if the handler was found, `false` otherwise.
 
 Remove a message handler previously registered to this channel with
 [`channel.subscribe(onMessage)`][].