[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/codecs.rst b/Doc/library/codecs.rst
index 24008a0..b9c04c2 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -1471,7 +1471,7 @@
 
 Encode operand according to the ANSI codepage (CP_ACP).
 
-Availability: Windows only.
+.. availability:: Windows only.
 
 .. versionchanged:: 3.3
    Support any error handler.