[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/using/cmdline.rst b/Doc/using/cmdline.rst
index f25e02b..dca89ec 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -769,7 +769,7 @@
    This may also be enabled at runtime with
    :func:`sys._enablelegacywindowsfsencoding()`.
 
-   Availability: Windows
+   .. availability:: Windows.
 
    .. versionadded:: 3.6
       See :pep:`529` for more details.
@@ -783,7 +783,7 @@
    This variable is ignored if the standard streams are redirected (to files
    or pipes) rather than referring to console buffers.
 
-   Availability: Windows
+   .. availability:: Windows.
 
    .. versionadded:: 3.6
 
@@ -834,7 +834,7 @@
    order to force the interpreter to use ``ASCII`` instead of ``UTF-8`` for
    system interfaces.
 
-   Availability: \*nix
+   .. availability:: \*nix.
 
    .. versionadded:: 3.7
       See :pep:`538` for more details.
@@ -895,7 +895,7 @@
 
    Also available as the :option:`-X` ``utf8`` option.
 
-   Availability: \*nix
+   .. availability:: \*nix.
 
    .. versionadded:: 3.7
       See :pep:`540` for more details.