Clarify that only required=True conflicts with default in field docs

[epuronta]

The fields docs say that "including both the default and required keyword arguments is invalid and will raise an error". The assertion in Field.__init__ is assert not (required and default is not empty), so only required=True combined with a default raises. An explicit required=False passes silently. Reword the note to match the implemented behavior.

[browniebroke]

Thanks