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.
We should create a validting embedded entities, as it's even more trickier than what we discuss here. See also https://github.com/symfony/symfony-docs/issues/3314