bpo-40975: [doc] Identify AsyncExitStack.enter_async_context()/aclose() as coroutine methods (GH-20870) (GH-26255)

(cherry picked from commit c054e8f)

Co-authored-by: naglis <naglis@users.noreply.github.com>

Author: miss-islington

Doc/library/contextlib.rst

@@ -463,7 +463,7 @@ Functions and classes provided:
    The :meth:`close` method is not implemented, :meth:`aclose` must be used
    instead.
 
-   .. method:: enter_async_context(cm)
+   .. coroutinemethod:: enter_async_context(cm)
 
       Similar to :meth:`enter_context` but expects an asynchronous context
       manager.
@@ -477,7 +477,7 @@ Functions and classes provided:
 
       Similar to :meth:`callback` but expects a coroutine function.
 
-   .. method:: aclose()
+   .. coroutinemethod:: aclose()
 
       Similar to :meth:`close` but properly handles awaitables.