Add a form_dict kwarg for calls to the done method of form wizards

This adds a ``form_dict`` keyword argument to calls to the ``done()`` method of FormWizard implementations. This specifically makes it easier to work with ``NamedUrlWizardViews`` as it allows refering to forms by their name rather than their index in the ``form_list`` list (which might change depending on ``condition_dict``).

We're also adding a missing ``**kwargs`` mention in the docs.