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.
I agree with you guys on merging this as quick and simply as possible. But I still don't understand how this feature works from this ``caution`` block. Let me suggest a wording, which may be technically incorrect - you can tell me please :).

```rst
    When validating *just* the User object, there is no difference between the ``Default`` group
    and the ``User`` group. But, there is a difference if ``User`` has embedded objects. For example,
    imagine ``User`` has an ``address`` property that contains some ``Address`` object and that
    you've added the :doc:`/reference/constraints/valid` constraint to this property so that it's
    validated when you validate the ``User`` object.

    If you validate ``User`` using the ``Default`` group, then any constraints on the ``Address``
    class that are in the ``Default`` group *will* be used. But, if you validate ``User`` using the
    ``User`` validation group, then only constraints on the ``Address`` class with the ``User``
    group will be validated.

    In other words, the ``Default`` group and the class name group (e.g. ``User``) are identical,
    except when the class is embedded in another object that's actually the one being validated.
```

Like it? Actually, I think I got it right, once I had to sit and think about how it works and use your sentences as my guide :).