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.
What about this?

```rst
Constraints in the ``Default`` group of a class are the constraints that have either no
explicit group configured or that are configured to a group equal to the class name or the string ``Default``.
```