bpo-40405: Fix asyncio.as_completed docs (GH-19753)

* Fix as_completed docs to correctly state the function return value.
* Also, improves the general wording of the as_completed documentation.

Co-Authored-By: Rémi Lapeyre <remi.lapeyre@henki.fr>
Co-Authored-By: Kyle Stanley <aeros167@gmail.com>
Co-Authored-By: Yury Selivanov <yury@edgedb.com>
(cherry picked from commit 13206b5)

Co-authored-by: Bar Harel <bzvi7919@gmail.com>

Doc/library/asyncio-task.rst

@@ -580,9 +580,9 @@ Waiting Primitives
 .. function:: as_completed(aws, \*, loop=None, timeout=None)
 
    Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
-   set concurrently.  Return an iterator of :class:`Future` objects.
-   Each Future object returned represents the earliest result
-   from the set of the remaining awaitables.
+   set concurrently.  Return an iterator of coroutines.
+   Each coroutine returned can be awaited to get the earliest next
+   result from the set of the remaining awaitables.
 
    Raises :exc:`asyncio.TimeoutError` if the timeout occurs before
    all Futures are done.
@@ -592,8 +592,8 @@ Waiting Primitives
 
    Example::
 
-       for f in as_completed(aws):
-           earliest_result = await f
+       for coro in as_completed(aws):
+           earliest_result = await coro
            # ...