[Validator] Updated documentation of URL validator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.7
| Fixed tickets | ~

Updated the documentation regarding the Pull Request on Symfony : https://github.com/symfony/symfony/pull/12956
you can do ``:phpfunction:`checkdnsrr` `` to link to the php.net page.