Documentation about `ClassDiscriminatorResolver` unclear

**Symfony version(s) affected**: 4.1

**Description**  
[Documentation](https://symfony.com/doc/master/components/serializer.html#serializing-interfaces-and-abstract-classes) mentions the use of `ClassDiscriminatorResolver` which is absent from Symfony 4.1 so either the docs need an update or `ClassDiscriminatorResolver` is missing from Symfony 4.1.