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.
Love it! We could even make it a cookbook article (it doesn't need to be long, only as long as it needs to be) - but a cookbook article makes it that much more "findable".

Ok, now that we've volunteered @peterrehm for more work, are you able to do it? Obviously, I realize the holidays are about start, so as much as I think this is a really important entry, it doesn't need to happen right away :).

Cheers!