bpo-34476: Document that asyncio.sleep() always suspends. (GH-9643) (#9654)

(cherry picked from commit cd602b8af2d14ff686261eeb18b80f718bb16550)

Co-authored-by: Hrvoje Nikšić <hniksic@gmail.com>
diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index 198bd7d..3168f47 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -270,6 +270,9 @@
    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.