[DOC] Make _fit_and_score return value docstring consistent with code.

The order of the return values currently listed in the cross_validation._fit_and_score docstring does not match the actual order of the return values when the kwarg `return_train_score=True`. In the code, this option causes the train_score to be the first return value, but in the docstring it's implied that it's the second.

This PR changes the docstring to be in sync with the code.