[Docs] undocumented method(s) on Object. #614

Looking at https://github.com/nodejs/node-addon-api/blob/master/doc/object.md

I failed to find a way to iterate over properties. I eventually found it via the napi docs and the addon implementation files.

Would be nice if the docs can be complete.

Napi::Object::GetPropertyNames() is not listed in the API docs.

Napi::Object::HasOwnProperty() is not listed in the API docs.

Napi::Object::Delete() is not listed in the API docs.