Customize: Clean up docs and code style for customize changes in 4.7.

* Adds missing `resolve`/`fail` for promise returned by `loadThemePreview`.
* Adds missing jsdoc blocks and tags.
* Adds missing phpdoc and makes corrections.

See #37770.
Fixes #38908.


git-svn-id: https://develop.svn.wordpress.org/trunk@39345 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

@@ -267,6 +267,7 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 	 * Get original title.
 	 *
 	 * @since 4.7.0
+	 * @access protected
 	 *
 	 * @param object $item Nav menu item.
 	 * @return string The original title.