Simple Documentation fix: Missing link to return type class. (GH-21291)

Just a simple documentation fix: apply_async and map_async return a "multiprocessing.pool.AsyncResult Object", not a "result object".
(cherry picked from commit f9bf015)

Co-authored-by: Volker-Weissmann <[email protected]>

Author: miss-islington

Doc/library/multiprocessing.rst

@@ -2163,7 +2163,8 @@ with the :class:`Pool` class.
 
    .. method:: apply_async(func[, args[, kwds[, callback[, error_callback]]]])
 
-      A variant of the :meth:`apply` method which returns a result object.
+      A variant of the :meth:`apply` method which returns a
+      :class:`~multiprocessing.pool.AsyncResult` object.
 
       If *callback* is specified then it should be a callable which accepts a
       single argument.  When the result becomes ready *callback* is applied to
@@ -2193,7 +2194,8 @@ with the :class:`Pool` class.
 
    .. method:: map_async(func, iterable[, chunksize[, callback[, error_callback]]])
 
-      A variant of the :meth:`.map` method which returns a result object.
+      A variant of the :meth:`.map` method which returns a
+      :class:`~multiprocessing.pool.AsyncResult` object.
 
       If *callback* is specified then it should be a callable which accepts a
       single argument.  When the result becomes ready *callback* is applied to