Add docstring to background update.

synapse/storage/databases/main/event_push_actions.py

@@ -249,6 +249,16 @@ def __init__(
     async def _background_backfill_thread_id(
         self, progress: JsonDict, batch_size: int
     ) -> int:
+        """
+        Fill in the thread_id field for event_push_actions and event_push_summary.
+
+        This is preparatory so that it can be made non-nullable in the future.
+
+        Because all current (null) data is done in an unthreaded manner this
+        simply assumes it is on the "main" timeline. Since event_push_actions
+        are periodically cleared it is not possible to correctly re-calculate
+        the thread_id.
+        """
         event_push_actions_done = progress.get("event_push_actions_done", False)
 
         def add_thread_id_txn(