Add WordPress Menu Item Description

add_filter( 'walker_nav_menu_start_el', 'gt_add_menu_item_description', 10, 4); 
function gt_add_menu_item_description( $item_output, $item, $depth, $args ) {
	$desc = __( $item->post_content ); 
	return preg_replace('/(<a.*?>[^<]*?)</', '$1' . "<small class=\"nav-desc\">{$desc}</small><", $item_output); 
}