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.
No, it always goes after the name of the group, and this is tricky. I want to raise awareness and in most cases it makes sense to use Default as group to ensure cascaded validation.