gh-132354: document return value for asyncio.Task.cancel (#132374)

Author: felixscherz

Doc/library/asyncio-task.rst

@@ -1381,7 +1381,10 @@ Task Object
 
       Request the Task to be cancelled.
 
-      This arranges for a :exc:`CancelledError` exception to be thrown
+      If the Task is already *done* or *cancelled*, return ``False``,
+      otherwise, return ``True``.
+
+      The method arranges for a :exc:`CancelledError` exception to be thrown
       into the wrapped coroutine on the next cycle of the event loop.
 
       The coroutine then has a chance to clean up or even deny the

Misc/ACKS

@@ -1663,6 +1663,7 @@ Andreas Schawo
 Neil Schemenauer
 David Scherer
 Wolfgang Scherer
+Felix Scherz
 Hynek Schlawack
 Bob Schmertz
 Gregor Schmid