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>