[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/c-api/init.rst b/Doc/c-api/init.rst
index 694b466..50998be 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -156,7 +156,7 @@
 
    See :pep:`529` for more details.
 
-   Availability: Windows.
+   .. availability:: Windows.
 
 .. c:var:: Py_LegacyWindowsStdioFlag
 
@@ -168,7 +168,7 @@
 
    See :pep:`528` for more details.
 
-   Availability: Windows.
+   .. availability:: Windows.
 
 .. c:var:: Py_NoSiteFlag