Fix docstring for limit argument in _maybe_backfill_inner(...) (#19630)

Incorrectly labeled in matrix-org/synapse#13535.

`maybe_backfill` already accurately describes `limit` (introduced in
matrix-org/synapse#8349)

Spotted in
#19611 (comment)

Author: MadLittleMods

changelog.d/19630.misc

@@ -0,0 +1 @@
+Fix docstring for `limit` argument in `_maybe_backfill_inner(...)`.

synapse/handlers/federation.py

@@ -242,7 +242,9 @@ async def _maybe_backfill_inner(
         Args:
             room_id: The room to backfill in.
             current_depth: The depth to check at for any upcoming backfill points.
-            limit: The max number of events to request from the remote federated server.
+            limit: The number of events that the pagination request will
+                return. This is used as part of the heuristic to decide if we
+                should back paginate.
             processing_start_time: The time when `maybe_backfill` started processing.
                 Only used for timing. If `None`, no timing observation will be made.