Abilities API: Improve inline documentation

This was discussed in #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 #10412.

Follw-up [61032].

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



git-svn-id: https://develop.svn.wordpress.org/trunk@61067 602fd350-edb4-49c9-b593-d223f7449a82

Author: gziolo