You know how you start pulling on a thread and before you notice you've unraveled the whole sweater?  I meant to do a simple *one line* fix on the docs and wound up with a nearly-300-line diff.  I claim it's all an improvement, though I'm willing to change it if there's something you don't like.

Definite improvements:
* supports_fd et al are :class:`set` objects, not :class:`~collections.Set`.  Those are different.
* Linked to the "specifying a file descriptor" section in os from the relevant discussion in what's new.
* "Availability: Unix" goes above :versionadded.

Avowed improvements:
* supports_* now explicitly mention what values are permitted for their parameters.
* Lots of rewriting/munging in an effort to improve readability and precision.
* A couple places where "it's" read better than "it is".

LGTU?