add docs

synapse/storage/e2e_room_keys.py

@@ -217,7 +217,10 @@ class EndToEndRoomKeyStore(SQLBaseStore):
         Raises:
             StoreError: with code 404 if there are no e2e_room_keys_versions present
         Returns:
-            A deferred dict giving the info metadata for this backup version
+            A deferred dict giving the info metadata for this backup version, with fields including:
+                version(str)
+                algorithm(str)
+                auth_data(object): opaque dict supplied by the client
         """
 
         def _get_e2e_room_keys_version_info_txn(txn):