Documentation

The .. availability:: directives in our documentation do not use a consistent syntax. The arguments are all over the place. For example versions are indicate as Linux >= 4.2, Linux 4.2 and newer, or Linux 4.2 or later. Some directives have additional content that point to man pages. Others reference minimum versions although we no longer support the version (e.g. FreeBSD 9 or later although FreeBSD 9 is not supported any more).

I would like to make the availability directive arguments consistent and parsable:

• Use consistent spelling for version restrictions platform >= version.
• Introduce not prefix to indicate that a feature is not available on that platform.
• Check for known platform strings and emit a warning when a platform is not in an allow list.
• Enable has_content for the directive and move all additional remarks to the contents body of the directive.

.. availability:: Windows, Linux >= 3.17, not WASI

   See :manpage:`python(1)` for more information

A consistent and parsable syntax will allow us to automatically generate a table of non-supported features on Emscripten and WASI later.