Fixed #8721: Clarification in docs for form previews

       :class:`~django.contrib.formtools.FormPreview` subclass::

           from myapp.preview import SomeModelFormPreview

           from myapp.models import SomeModel

           from myapp.forms import SomeModelForm

           from django import forms

       ...and add the following line to the appropriate model in your URLconf::

=========================

By default, the form is rendered via the template :file:`formtools/form.html`,

and the preview page is rendered via the template :file:`formtools.preview.html`.

and the preview page is rendered via the template :file:`formtools/preview.html`.

These values can be overridden for a particular form preview by setting

:attr:`~django.contrib.formtools.FormPreview.preview_template` and

:attr:`~django.contrib.formtools.FormPreview.form_template` attributes on the