Add menu item description to a menu item

functions.php

//
// Add the menu item description to the visible title.
//

add_filter( 'nav_menu_item_title', 'nav_add_desc', 10, 4);

function nav_add_desc( $title, $menu_item,  $args, $depth ) {
	$retHtml = '';

	$desc = $menu_item->description;
	if ( strlen($desc) > 0) {
		$desc = '<span>'.$desc.'</span>';
	}

	return $title.$desc;
}