[3.1] Document Controller#json() helper

[wouterj]

Thanks to @mcfedr, Symfony 3.1 comes with a sweet little Controller#json() helper to create a JsonResponse using the Symfony serializer (or just using the plain values). As we slowly start to show more and more how to use Symfony as an API server, I think it's worth documenting this method (especially in the Controller book chapter).

Refs symfony/symfony#17642

[wouterj]

Done by #6502