Update ValidationListener.php

This comment was misleading.
It's true that no groups (ie.: `Default`) are setted into validation process but it's not clear from the comment.
I suppose that is better to be more explicit about this process.
BTW I'm still not sure if `FormValidator` reference is necessary as we can change it and we should remember to update this comment and this does not seems to fit.
WDYT, is better to leave `FormValidator` out of this comment?

For reference: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php#L46-L92

src/Symfony/Component/Form/Extension/Validator/EventListener/ValidationListener.php

@@ -51,7 +51,7 @@ class ValidationListener implements EventSubscriberInterface
         $form = $event->getForm();
 
         if ($form->isRoot()) {
-            // Validate the form in group "Default"
+            // Form groups are validated internally (FormValidator). Here we don't set groups as they are retrieved into the validator.
             foreach ($this->validator->validate($form) as $violation) {
                 // Allow the "invalid" constraint to be put onto
                 // non-synchronized forms