add MenuOptionsInterface docs

this should also have an XML and PHP example:

````xml
<!-- app/config/config.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://cmf.symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <config xmlns="http://cmf.symfony.com/schema/dic/menu">
        <admin-extensions>
            <menu-options advanced="true">
        </admin-extensions>
    </config>

</container>
````

PHP:

````php
// app/config/config.php
$container->loadFromExtension('cmf_menu', array(
    'admin_extensions' => array(
        'menu_options' => array(
            'advanced' => true,
        ),
    ),
));
````