[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/threading.rst b/Doc/library/threading.rst
index e6185c5..a9d5268 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -100,7 +100,8 @@
    memory page size - platform documentation should be referred to for more
    information (4 KiB pages are common; using multiples of 4096 for the stack size is
    the suggested approach in the absence of more specific information).
-   Availability: Windows, systems with POSIX threads.
+
+   .. availability:: Windows, systems with POSIX threads.
 
 
 This module also defines the following constant: