[3.12] gh-108951: Document how to terminate an asyncio.TaskGroup (GH-123837) (#123957)

gh-108951: Document how to terminate an asyncio.TaskGroup (GH-123837)

We don't want to add another API, since the recipe is straightforward and rarely needed.

The advantage is that we could backport this to the earliest Python version that has taskgroups (3.11, alas in security mode already, so we'll just do 3.12 and 3.13).
(cherry picked from commit ef05801ba0cbf090034df17e2a0420fb42c2d538)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>