Add comment describing why we do not answer make_joins

synapse/handlers/federation.py

@@ -752,6 +752,10 @@ async def on_make_join_request(
             # If our server is still only partially joined, we can't give a complete
             # response to /make_join, so return a 404 as we would if we weren't in the
             # room at all.
+            # The main reason we can't respond properly is that we need to know about
+            # the auth events for the join event that we would return.
+            # Another reason is that we should not entertain the make_join since we
+            # cannot handle the send_join.
             logger.info(
                 "Rejecting /make_join to %s because it's a partial state room", room_id
             )