[3.14] gh-134664: document `cleanup_socket` parameter in `asyncio.start_unix_server` (GH-134750) (#134779)

gh-134664: document `cleanup_socket` parameter in `asyncio.start_unix_server` (GH-134750)
(cherry picked from commit 92ea1eb38ff97ac046a0031d505c30a51f58a43f)

Co-authored-by: Shamil <ashm.tech@proton.me>