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.
Can we maybe shrink this down to something like:

"There is a substantial difference between the ``Default`` and ``User`` group. The ``Default`` group includes all constraints of the current entity (``User``) *and* all entities validated using the Valid constraint, while ``User`` only includes the constraints of the current entity."

(with "Valid constraint" being a link to the Valid constraint reference docs)