gh-108951: add TaskGroup.cancel()

Doc/library/asyncio-task.rst

@@ -359,6 +359,14 @@ and reliable way to wait for all tasks in the group to finish.
       :meth:`cancel` is idempotent and may be called after the task group has
       already exited.
 
+      Ways to use :meth:`cancel`:
+
+      * call it from the task group body based on some condition or event
+      * pass the task group instance to child tasks via :meth:`create_task`, allowing a child
+        task to conditionally cancel the entire entire group
+      * pass the task group instance or bound :meth:`cancel` method to some other task *before*
+        opening the task group, allowing remote cancellation
+
       .. versionadded:: next
 
 Example::