Abilities API: Improve inline documentation

This was discussed in WordPress/wordpress-develop#9410 and decided to defer until after beta1.

Fleshing out the phpdocs for the extender functions in https://core.trac.wordpress.org/browser/trunk/src/wp-includes/abilities-api.php should make it easier for people working with the abilities API

Developed in WordPress/wordpress-develop#10412.

Follw-up [61032].

Props jorbin, gziolo, mukesh27, jason_the_adams, westonruter.
Fixes #64134.


Built from https://develop.svn.wordpress.org/trunk@61067


git-svn-id: https://core.svn.wordpress.org/trunk@60403 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: gziolo