HV-900 Documenting Java 8 support

... of a parameterized type. However, this requires that `ElementType.TYPE_USE` is specified via `@Target` in the constraint definition. Due to backwards compatibility reasons the Bean Validation constraints as well as the Hibernate Validator specific ones, do not yet specify  `ElementType.TYPE_USE`. To make use of type arguments constraints custom constraints must be used for now... 

Something like that. Also, don't mention that we might provide two sets of constraints.