Docs: Add missing parameter and return descriptions for `Walker::get_number_of_root_elements()`.

Fixes #33662.


git-svn-id: https://develop.svn.wordpress.org/trunk@34121 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -387,8 +387,8 @@ class Walker {
 	 * @since 2.7.0
 	 * @access public
 	 * 
-	 * @param array $elements
-	 * @return int
+	 * @param array $elements Elements to list.
+	 * @return int Number of root elements.
 	 */
 	public function get_number_of_root_elements( $elements ){
 		$num = 0;