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.
So this stuff is really important, and really confusing - I'm convinced that @peterrehm (and probably also @WouterJ) understand it better than I do. When I think about how embedded things work and this Default versus the class name group stuff, I get sleepy :).

My point is this: it's important and tricky enough. So, what do you think about making another section after this (e.g. `Validation Groups in Embedded Entities` or event `Validating Embedded Entities` that would also show `@Valid`) that shows concrete examples. 

What do you think?