[3.14] gh-148091: clarify asyncio.Future.cancel(msg) behaviour (GH-148248) (#148299)

gh-148091: clarify asyncio.Future.cancel(msg) behaviour (GH-148248)
(cherry picked from commit 2acb8d9257c4f5049777d9d462092373b0b3feca)

Co-authored-by: Manoj K M <136242596+manoj-k-m@users.noreply.github.com>