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.


git-svn-id: https://develop.svn.wordpress.org/trunk@29843 602fd350-edb4-49c9-b593-d223f7449a82