[3.7] bpo-11233: Create availability directive for documentation (GH-9692) (GH-9830)

Replace "Availability: xxx" with ".. availability:: xxx" in the doc.
 Original patch by Georg Brandl.

 Co-Authored-By: Georg Brandl <georg@python.org>
 (cherry picked from commit 2d6097d027e0dd3debbabc702aa9c98d94ba32a3)

 Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
diff --git a/Doc/library/asyncio-policy.rst b/Doc/library/asyncio-policy.rst
index 42f936d..cab2593 100644
--- a/Doc/library/asyncio-policy.rst
+++ b/Doc/library/asyncio-policy.rst
@@ -103,7 +103,7 @@
    An alternative event loop policy that uses the
    :class:`ProactorEventLoop` event loop implementation.
 
-   Availability: Windows.
+   .. availability:: Windows.
 
 
 Process Watchers