[Validation] Interpreting String as Date in EqualTo Validators

[timglabisch]

We should document, that the EqualTo Validators can interprete strings as Dates / DateTimes

Document Here
http://symfony.com/doc/current/reference/constraints/EqualTo.html

The Tests:
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Validator/Tests/Constraints/EqualToValidatorTest.php#L49

[xabbuh]

@timglabisch Thank you for reporting this. You are right that the documentation is currently missing this feature. However, as #4143 will also cover this, I'm going to close this since we already have issue #4426.