Caution that roles should start with ROLE_

In order to save someone else a few hours of frustration make it clear that the ROLE_ preface is not just for show.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |
@stof Well, I thought that at least http://symfony.com/doc/current/book/security.html#roles and its note are correct.