Add descriptions to navigation items

functions.php

<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* Add description to menu items
add_filter( 'walker_nav_menu_start_el', 'wpstudio_add_description', 10, 2 );
function wpstudio_add_description( $item_output, $item ) {

    $description = $item->post_content;
    if (' ' !== $description ) {
        return preg_replace( '/(<a.*)</', '$1' . '<span class="menu-description">' . $description . '</span><', $item_output) ;
    }
    else {
        return $item_output;
    };

}