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.
Ah, seems like I didn't finish the first part of my sentence. I indeed meant a new cookbook article.

Btw, I believe we should merge this PR first and then move them all to a cookbook, as it will require some time till we get all things in that cookbook correct.