Clarify empty responses

proposals/2666-get-rooms-in-common.md

@@ -22,7 +22,8 @@ Homeservers should implement a new endpoint `/users/{current_user}/shared_rooms/
 the authenticated users MxID and the user that is being searched for.
 
 The response format will be an array containing all rooms where both the `current_user` and `other_user_id` have
-a membership of type `join`. 
+a membership of type `join`. If either user is not joined to any rooms, or the `other_user_id` does not exist, an
+empty array should be returned.
 
 ```
 GET _matrix/client/unstable/users/@alice:example.com/shared_rooms/@bob:example.com