Add a comment which might save some confusion

synapse/app/synchrotron.py

@@ -340,6 +340,7 @@ class SyncReplicationHandler(ReplicationClientHandler):
 
         self.store = hs.get_datastore()
         self.typing_handler = hs.get_typing_handler()
+        # NB this is a SynchrotronPresence, not a normal PresenceHandler
         self.presence_handler = hs.get_presence_handler()
         self.notifier = hs.get_notifier()