1. Changed function wp_get_post_object_type() to method of WP_Metadata and renamed to ::get_post_object_type_literal().
2. Added PHPDoc to WP_Object_Type
3. Updated PHPDoc for WP_Html_Element
4. Added PHPDoc to function register_object_type_class().
5. Added WP_Metadata::sanitize_identifier() and used in both WP_Object_Type and WP_Html_Element.