Inline documentation cleanup for 4.0 audit.

phpDoc tweaks for methods added in [28861]:
* `WP_Customize_Manager::panels()` method
* `WP_Customize_Manager::add_panel()` method
* `WP_Customize_Manager::get_panel()` method
* `WP_Customize_Manager::remove_panel()` method

Added in [28970]:
* `WP_Customize_Manager::customize_preview_override_404_status()` method

See #27406 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29157


git-svn-id: http://core.svn.wordpress.org/trunk@28941 1a063a9b-81f0-0310-95a4-ce76da25c4cd