ENUNCIATE-785: correct reference to "jsonElement" in docs.xml

This bug is caused by two error in sequence:
1. the element is never generated in docs.xml, because the ResourceRepresentationMetadata is not supported datatype descriptor (it's actually just wrapper around TypeMirror, so it's easy to fix)
2. generated name is jsonElement, but docs.fmt references jsonElementRef (which is probably correct, because this name us used by input arguments)