PHPDoc blocks of register_widget and unregister_widget are not up-to-date

[behzod]

register_widget and unregister_widget functions can accept either the name of a WP_Widget subclass or an instance of a WP_Widget subclass since 4.6.0 (see WP_Widget_Factory::register() and WP_Widget_Factory::unregister()) but the PHPDoc blocks are not up-to-date.

Attaching a patch that improves the documentation. It also refactors functions per WP coding standards.

[SergeyBiryukov]

In 41942:

Docs: After [37329], update the documentation for register_widget() and unregister_widget().

Both functions can accept a WP_Widget instance object instead of a WP_Widget subclass name, same as WP_Widget_Factory::register() and ::unregister().

Props behzod.
Fixes #42270.