Possible for register to use factory docstring

[michaeloliverx]

Using the convenience register method to register fixtures does not import the docstring. Running pytest --fixtures results in no docstring available for all pytest-factoryboy fixtures.
It would be nice to include a docstring arg to the register method e.g

register(UserFactory, _name="special_user", _docstring="Useful sentence describing this fixture")

Thanks for the library it is very handy!