added docstring

google/cloud/bigtable/_mutate_rows.py

@@ -32,6 +32,25 @@ async def _mutate_rows_retryable_attempt(
     mutation_dict: dict[int, "BulkMutationsEntry"|None],
     error_dict: dict[int, list[Exception]],
 ):
+    """
+    Helper function for managing the mutate_rows lifecycle.
+
+    Function will continuously retry failed mutations, until all mutations succeed or encounter
+    an unrecoverable error.
+
+    This function is intended to be wrapped in an api_core.retry.AsyncRetry object, which will handle
+    timeouts and retrying raised exceptions.
+
+    Args:
+      - gapic_client: the client to use for the mutate_rows call
+      - request: the request to send to the server, populated with table name and app profile id
+      - per_request_timeout: the timeout to use for each mutate_rows attempt
+      - mutation_dict: a dictionary tracking which entries are outstanding 
+            (stored as BulkMutationsEntry), and which have reached a terminal state (stored as None).
+            At the start of the request, all entries are outstanding.
+      - error_dict: a dictionary tracking errors associated with each entry index.
+            Each retry will append a new error. Successful mutations will clear the error list.
+    """
     new_request = request.copy()
     while any(mutation is not None for mutation in mutation_dict.values()):
         await asyncio.sleep(0)