bpo-34476: Document that asyncio.sleep() always suspends. (#9643)

Author: hniksic

Doc/library/asyncio-task.rst

@@ -276,6 +276,9 @@ Sleeping
    If *result* is provided, it is returned to the caller
    when the coroutine completes.
 
+   ``sleep()`` always suspends the current task, allowing other tasks
+   to run.
+
    The *loop* argument is deprecated and scheduled for removal
    in Python 3.10.