document min, max kwargs to traitlets.{Int,Float}

The `min` and `max` kwargs are "manually" implemented as kw-only args to `Int` and `Float`. As a consequence, they do not appear in the generated docs (https://traitlets.readthedocs.io/en/stable/trait_types.html#numbers) anywhere AFAICT. I think sphinx allows manually overriding the signature to e.g. use the Python3-format (`Int(default_value=Undefined, allow_none=False, *, min=None, max=None, **kwargs)`) in the docs.