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 |
I would not say ``The default role voter``. the main confusion actually comes from a wrong usage of the term ``role`` in the documentation. See https://github.com/symfony/symfony-docs/issues/4158