Added information about the new date handling in the comparison constraints and Range

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11673)
| Applies to    | 2.6+
| Fixed tickets | -
We already use different examples in the constraint reference (e.g. [AcmeBlogBundle](http://symfony.com/doc/current/reference/constraints/NotBlank.html#basic-usage), [AcmeEventBundle](http://symfony.com/doc/current/reference/constraints/Range.html#basic-usage), and so on). So, I think that's okay here.