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

Doc/library/asyncio-task.rst

@@ -414,6 +414,53 @@ reported by :meth:`asyncio.Task.cancelling`.
    Improved handling of simultaneous internal and external cancellations
    and correct preservation of cancellation counts.
 
+Terminating a Task Group
+------------------------
+
+While terminating a task group is not natively supported by the standard
+library, termination can be achieved by adding an exception-raising task
+to the task group and ignoring the raised exception:
+
+.. code-block:: python
+
+   import asyncio
+   from asyncio import TaskGroup
+
+   class TerminateTaskGroup(Exception):
+       """Exception raised to terminate a task group."""
+
+   async def force_terminate_task_group():
+       """Used to force termination of a task group."""
+       raise TerminateTaskGroup()
+
+   async def job(task_id, sleep_time):
+       print(f'Task {task_id}: start')
+       await asyncio.sleep(sleep_time)
+       print(f'Task {task_id}: done')
+
+   async def main():
+       try:
+           async with TaskGroup() as group:
+               # spawn some tasks
+               group.create_task(job(1, 0.5))
+               group.create_task(job(2, 1.5))
+               # sleep for 1 second
+               await asyncio.sleep(1)
+               # add an exception-raising task to force the group to terminate
+               group.create_task(force_terminate_task_group())
+       except* TerminateTaskGroup:
+           pass
+
+   asyncio.run(main())
+
+Expected output:
+
+.. code-block:: text
+
+   Task 1: start
+   Task 2: start
+   Task 1: done
+
 Sleeping
 ========