[Reference] document the `2.5` validation options

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.5+
| Fixed tickets | part of #4094
If I correctly understand the changes made in symfony/symfony#12975, we will have to change this description for Symfony 2.5 and higher since the value of the `cache` option is now treated as the name of the service.