[3.14] gh-148091: clarify asyncio.Future.cancel(msg) behaviour (GH-148248) (#148299)

gh-148091: clarify asyncio.Future.cancel(msg) behaviour (GH-148248)
(cherry picked from commit 2acb8d9)

Co-authored-by: Manoj K M <136242596+manoj-k-m@users.noreply.github.com>

Author: miss-islington

Doc/library/asyncio-future.rst

@@ -196,6 +196,10 @@ Future Object
       Otherwise, change the Future's state to *cancelled*,
       schedule the callbacks, and return ``True``.
 
+      The optional string argument *msg* is passed as the argument to the
+      :exc:`CancelledError` exception raised when a cancelled Future
+      is awaited.
+
       .. versionchanged:: 3.9
          Added the *msg* parameter.