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.

git-svn-id: https://develop.svn.wordpress.org/trunk@27543 602fd350-edb4-49c9-b593-d223f7449a82