Docs: Add missing documentation for some Customize class properties:

* `WP_Customize_Background_Image_Control::$type`
* `WP_Customize_Background_Image_Setting::$id`
* `WP_Customize_Header_Image_Setting::$id`

Follow-up to [21037], [21053], [30885].

Props iamjaydip.
See #55646.

git-svn-id: https://develop.svn.wordpress.org/trunk@53411 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-background-image-control.php

@@ -15,6 +15,13 @@
  * @see WP_Customize_Image_Control
  */
 class WP_Customize_Background_Image_Control extends WP_Customize_Image_Control {
+
+	/**
+	 * Customize control type.
+	 *
+	 * @since 4.1.0
+	 * @var string
+	 */
 	public $type = 'background';
 
 	/**

src/wp-includes/customize/class-wp-customize-background-image-setting.php

@@ -15,6 +15,13 @@
  * @see WP_Customize_Setting
  */
 final class WP_Customize_Background_Image_Setting extends WP_Customize_Setting {
+
+	/**
+	 * Unique string identifier for the setting.
+	 *
+	 * @since 3.4.0
+	 * @var string
+	 */
 	public $id = 'background_image_thumb';
 
 	/**

src/wp-includes/customize/class-wp-customize-header-image-control.php

@@ -15,6 +15,7 @@
  * @see WP_Customize_Image_Control
  */
 class WP_Customize_Header_Image_Control extends WP_Customize_Image_Control {
+
 	/**
 	 * Customize control type.
 	 *

src/wp-includes/customize/class-wp-customize-header-image-setting.php

@@ -17,6 +17,13 @@
  * @see WP_Customize_Setting
  */
 final class WP_Customize_Header_Image_Setting extends WP_Customize_Setting {
+
+	/**
+	 * Unique string identifier for the setting.
+	 *
+	 * @since 3.4.0
+	 * @var string
+	 */
 	public $id = 'header_image_data';
 
 	/**