add MenuOptionsInterface docs

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | dev
| Fixed tickets | https://github.com/symfony-cmf/MenuBundle/pull/197
the reference for the menu configuration is this file: https://github.com/symfony-cmf/symfony-cmf-docs/blob/master/reference/configuration/menu.rst . i would put the admin_extensions part right before the voter part. (note that the github rendering of the .rst files eats away half of the content - look at the source or the rendered page at http://symfony.com/doc/master/cmf/reference/configuration/menu.html )