Widget Customizer: Make temp hooks final and add inline docs.

New hooks are `dynamic_sidebar_before`, `dynamic_sidebar_after`, `dynamic_sidebar_has_widgets ` and `is_active_sidebar`.
Remove obsolete use of hacky dynamic_sidebar hook.

props westonruter, DrewAPicture.
fixes #25368.
Built from https://develop.svn.wordpress.org/trunk@27543


git-svn-id: http://core.svn.wordpress.org/trunk@27386 1a063a9b-81f0-0310-95a4-ce76da25c4cd