Add a comment in _process_received_pdu (#11011)

changelog.d/11011.misc

@@ -0,0 +1 @@
+Clean up some of the federation event authentication code for clarity.

synapse/handlers/federation_event.py

@@ -894,6 +894,9 @@ class FederationEventHandler:
                 backfilled=backfilled,
             )
         except AuthError as e:
+            # FIXME richvdh 2021/10/07 I don't think this is reachable. Let's log it
+            #   for now
+            logger.exception("Unexpected AuthError from _check_event_auth")
             raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)
 
         await self._run_push_actions_and_persist_event(event, context, backfilled)