Docs: Add a missing summary to the DocBlock for Walker::get_number_of_root_elements().

Props wen-solutions
See #33662.


git-svn-id: https://develop.svn.wordpress.org/trunk@33868 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -382,7 +382,11 @@ class Walker {
 	}
 
 	/**
+	 * Calculates the total number of root elements.
 	 *
+	 * @since 2.7.0
+	 * @access public
+	 * 
 	 * @param array $elements
 	 * @return int
 	 */