refine the availability

Author: pxinwr

Doc/library/netrc.rst

@@ -14,8 +14,8 @@
 
 This module implements an object-oriented interface to :file:`.netrc` files.
 
-VxWorks has no user home directory so it doesn't support ftp auto-login. This module
-is not supported on VxWorks.
+.. availability:: Unix. VxWorks has no user home directory so it doesn't support ftp
+auto-login. This module is not supported on VxWorks.
 
 The :class:`~netrc.netrc` class parses and encapsulates the netrc file format used by
 the Unix :program:`ftp` program and other FTP clients.