[3.13] gh-119711: describe `create_datagram_endpoint`'s behavior in asyncio when `local_addr` is `None` (GH-136913) (#137163)

gh-119711: describe `create_datagram_endpoint`'s behavior in asyncio when `local_addr` is `None` (GH-136913)
(cherry picked from commit 1481384141342479b3ba4b89f653b4e5bef0d272)

Co-authored-by: AN Long <aisk@users.noreply.github.com>