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.
Hmm, I think we should also mention the case of extending: https://github.com/symfony/symfony/issues/11880#issuecomment-60369867