Docs: Add a @global entry for $wpdb in WP_Comment_Query::fill_descendants().

Props jontyravi.
Fixes #41412.

git-svn-id: https://develop.svn.wordpress.org/trunk@41123 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -935,6 +935,8 @@ class WP_Comment_Query {
 	 * the descendant trees for all matched top-level comments.
 	 *
 	 * @since 4.4.0
+	 * 
+	 * @global wpdb $wpdb WordPress database abstraction object.
 	 *
 	 * @param array $comments Array of top-level comments whose descendants should be filled in.
 	 * @return array