Modify doc for napi_define_class to be less C++-specific

The doc for napi_define_class pretty much assumes an ObjectWrap scenario in that a C++ class will be associated one-to-one with the JS class being defined. This is not necessarily and always the case. We should limit the doc to what can be done in C.

... or at least de-emphasize the ObjectWrap use case.

doc: de-emphasize wrapping in napi_define_class