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 reword this a bit and also link to the "Roles" section in the security chapter of the book:

```rst
.. caution::

    The default role voter requires to prefix all roles with ``ROLE_`` (see
    the :ref:`section about roles <book-security-roles>` in the book). For
    example, your roles will be ``ROLE_ADMIN`` or ``ROLE_USER`` instead of
    ``ADMIN`` or ``USER``.
```

In ``/book/security.rst`` you would then have to change it to something like this (add the label before the headline):

```rst
.. _book-security-roles:

Roles
-----
```