Docs: Add a summary, version, and @access tag to the property DocBlock for Custom_Background->updated.

See [13765] for where the property was initially introduced.
See [30186] for where the property was actually declared.

Props brentvr.
See #30224. See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@34490 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-background.php

@@ -32,6 +32,10 @@ class Custom_Background {
 	public $admin_image_div_callback;
 
 	/**
+	 * Used to trigger a success message when settings updated and set to true.
+	 *
+	 * @since 3.0.0
+	 * @access private
 	 * @var bool
 	 */
 	private $updated;