Document the trans_default_domain Twig tag

I just re-discovered the `{% trans_default_domain %}` tag [available in the TransExtension](https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Extension/TranslationExtension.php#L68)

This gem should be documented (it is a 2.1 feature). See symfony/symfony#4944
