Add inline documentation for `WP_User_Query` default arguments in the form of a hash notation.

Adds documentation pointers from the class-level doc for `WP_User_Query`, as well as the `get_users()` doc.

Props tschutter.
Fixes #29846.

Built from https://develop.svn.wordpress.org/trunk@29843


git-svn-id: http://core.svn.wordpress.org/trunk@29607 1a063a9b-81f0-0310-95a4-ce76da25c4cd