Clarified Default vs ClassName groups
Updated after review of @webmozart
bug #3593 Clarified Default and ClassName groups (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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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 | -

Commits
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b6357fa Updated after review of @webmozart
2330cd8 Clarified Default vs ClassName groups