gh-141186: Document asyncio Task cancellation propagation behavior

Clarifies that cancelling a Task awaiting another Task or Future will
also cancel the awaited object. This behavior was undocumented despite
being fundamental to asyncio's cancellation architecture.

Changes:
- Updated general Task description to mention Task cancellation propagation
- Added explicit explanation in Task.cancel() method documentation
- Clarified that Tasks inherit Future's cancellation behavior

Addresses issue #141186 where users were unaware this cancellation
propagation was intentional architectural behavior, not a side effect.

The fix uses the exact wording suggested by the issue reporter and
documents the _fut_waiter implementation behavior that enables the
propagation down entire await chains.

Author: mohsinm-dev

Doc/library/asyncio-task.rst

@@ -1221,8 +1221,8 @@ Task Object
 
    To cancel a running Task use the :meth:`cancel` method.  Calling it
    will cause the Task to throw a :exc:`CancelledError` exception into
-   the wrapped coroutine.  If a coroutine is awaiting on a Future
-   object during cancellation, the Future object will be cancelled.
+   the wrapped coroutine.  If a coroutine is awaiting on a future-like
+   object during cancellation, the awaited object will be cancelled.
 
    :meth:`cancelled` can be used to check if the Task was cancelled.
    The method returns ``True`` if the wrapped coroutine did not
@@ -1231,7 +1231,8 @@ Task Object
 
    :class:`asyncio.Task` inherits from :class:`Future` all of its
    APIs except :meth:`Future.set_result` and
-   :meth:`Future.set_exception`.
+   :meth:`Future.set_exception`. This includes the cancellation
+   behavior: Tasks can be cancelled in the same way as Futures.
 
    An optional keyword-only *context* argument allows specifying a
    custom :class:`contextvars.Context` for the *coro* to run in.
@@ -1411,6 +1412,10 @@ Task Object
       the cancellation, it needs to call :meth:`Task.uncancel` in addition
       to catching the exception.
 
+      If the Task being cancelled is currently awaiting a future-like
+      object, that awaited object will also be cancelled. This cancellation
+      propagates down the entire chain of awaited objects.
+
       .. versionchanged:: 3.9
          Added the *msg* parameter.