Try to better explain why

See https://github.com/element-hq/synapse/pull/17293#discussion_r1633904606

synapse/storage/databases/main/stream.py

@@ -914,11 +914,15 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         def get_last_event_in_room_before_stream_ordering_txn(
             txn: LoggingTransaction,
         ) -> Optional[str]:
-            # We need to handle the fact that the stream tokens can be vector clocks. We
-            # do this by getting all rows between the minimum and maximum stream
-            # ordering in the token, plus one row less than the minimum stream ordering
-            # (TODO: Why?). We then filter the results against the token and return the
-            # first row that matches.
+            # We're looking for the closest event at or before the token. We need to
+            # handle the fact that the stream token can be a vector clock (with an
+            # `instance_map`) and events can be persisted on different instances
+            # (sharded event persisters). The first subquery handles the events that
+            # would be within the vector clock and gets all rows between the minimum and
+            # maximum stream ordering in the token which need to be filtered against the
+            # `instance_map`. The second subquery handles the "before" case and finds a
+            # row before the token. We then filter out any results past the token's
+            # vector clock and return the first row that matches.
             min_stream = end_token.stream
             max_stream = end_token.get_max_stream_pos()