Ideally, I prefer to stick with either a separate customization step or with the original __new__ constructor, and that uses **kwds so we can use a standard syntax for the name value pairs and to allow that possibility of someone passing in an existing dict using NT.set_docstrings(**d).

Also FWIW, the addition can be simplified a bit when Issue 24064 is added:

        for fieldname, docstring in docstrings.items():
            getattr(cls, fieldname).__doc__ = docstring