Improve documentation on start_background_task() function

src/flask_socketio/__init__.py

@@ -690,9 +690,9 @@ def start_background_task(self, target, *args, **kwargs):
         :param args: arguments to pass to the function.
         :param kwargs: keyword arguments to pass to the function.
 
-        This function returns an object compatible with the `Thread` class in
-        the Python standard library. The `start()` method on this object is
-        already called by this function.
+        This function returns an object that represents the background task,
+        on which the ``join()`` methond can be invoked to wait for the task to
+        complete.
         """
         return self.server.start_background_task(target, *args, **kwargs)