Doc/tools/extensions/availability.py defines the allowed platforms.

Whilst we're editing this, please could you also update the headers to use sentence case?

"Edge and level trigger polling (epoll) objects" and so on.

https://devguide.python.org/documentation/style-guide/#capitalization

Also in this table: https://docs.python.org/3/library/select.html#edge-and-level-trigger-polling-epoll-objects

• Change "assoc. fd" to "associated fd" or "associated file descriptor"
• "polling on a fd" -> "polling on an fd" or "polling on a file descriptor"
• Some of the Meaning column entries end with a full stop, some don't. Let's add them to all.

Elsewhere:

• "process id" -> "process ID"
• "Register a fd descriptor" -> "Register a file descriptor"
• "Filter specific" -> "Filter-specific"
• "User defined value" -> "User-defined value"