Add a return section to the docstring

Author: cpsievert

shiny/ui/_chat.py

@@ -555,6 +555,13 @@ async def append_message_stream(self, message: Iterable[Any] | AsyncIterable[Any
         ----
         Use this method (over `.append_message()`) when `stream=True` (or similar) is
         specified in model's completion method.
+
+        Returns
+        -------
+        reactive.ExtendedTask
+            An extended task that represents the streaming task. The `.result()` method
+            of the task can be called in a reactive context to get the final state of the
+            stream.
         """
 
         message = _utils.wrap_async_iterable(message)