Clarified Default and ClassName groups

This is still the most difficult part of the validation groups and we
don't put much attention on it. I've refactored it a bit after the
really great explaination of @webmozart.

For more information see: https://github.com/symfony/symfony/pull/10287#discussion_r9827190

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | yes
| Applies to | all
| Fixed tickets | -