Add inline documentation for the `option_none_value` argument added to `wp_dropdown_users|categories()` in [28564].

Fixes #16625.

Built from https://develop.svn.wordpress.org/trunk@28570


git-svn-id: http://core.svn.wordpress.org/trunk@28395 1a063a9b-81f0-0310-95a4-ce76da25c4cd