[3.7] bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217...

[3.7] bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217) (#7227)

* bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217)

Unlike `asyncio.wait_for()`, `asyncio.wait()` does not cancel the passed
futures when a timeout accurs.
(cherry picked from commit f9aeca2085464838f04bf13f816a1f861d43541f)

Co-authored-by: Elvis Pranskevichus <elvis@magic.io>
* Update asyncio-task.rst