More concrete explanation of validation groups

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Improved the explanation of default validation groups in reference with my discussion with @webmozart in symfony/symfony#11880.
Wouldn't it be easier to replace lines 569 to 577 by something like this?

```php
if (Entity\Client::TYPE_PERSON == $data->getType()) {
    return array('Default', 'person');
}

return array('Default', 'company');
```