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.
either remove the full stop or use `.. code-block:: php` instead of the shorthand. This will now be rendered as `as follows.:` which isn't that nice (I have to admit that this is a very minor detail).