document some ways to use cancel()

Author: belm0

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::