Flesh out and fix formatting in the DocBlock for `wp_register_widget_control()`.

Includes:
* Added todos for documenting `$options` in the hash notation style
* Backtick-escaping and general formatting

See #30315.


git-svn-id: https://develop.svn.wordpress.org/trunk@30778 602fd350-edb4-49c9-b593-d223f7449a82