bpo-31033: Add a msg argument to Future.cancel() and Task.cancel() (GH-19979)

diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index bc8a272..2e96339 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -724,7 +724,7 @@
    .. deprecated-removed:: 3.8 3.10
       The *loop* parameter.
 
-   .. method:: cancel()
+   .. method:: cancel(msg=None)
 
       Request the Task to be cancelled.
 
@@ -739,6 +739,9 @@
       suppressing cancellation completely is not common and is actively
       discouraged.
 
+      .. versionchanged:: 3.9
+         Added the ``msg`` parameter.
+
       .. _asyncio_example_task_cancel:
 
       The following example illustrates how coroutines can intercept