Docs: Add missing @var tag for WP_Query::$query_vars_changed.

Follow-up to [17552].

Props marian1, jigar-bhanushali, martin.krcho, parthvataliya.
Fixes #62022.

git-svn-id: https://develop.svn.wordpress.org/trunk@59495 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-query.php

@@ -442,6 +442,7 @@ class WP_Query {
 	 * via pre_get_posts hooks.
 	 *
 	 * @since 3.1.1
+	 * @var bool
 	 */
 	private $query_vars_changed = true;