[Reference] add validation config reference section

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
I think this was added in 2.5: https://github.com/symfony/symfony/blob/2.3/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php#L375