Missing documentation for FormPreview methods

[Honza Král]

The FormPreview class has a lot of methods that enable user's to modify it's default behavior. Most of these methods are not documented.

​http://docs.djangoproject.com/en/dev/ref/contrib/formtools/form-preview/#advanced-formpreview-methods

[Gabriel Hurley]

There are a number of problems with the docs for FormPreview...

1. They're badly organized. All the attribute and method reference documentation for FormPreview should live together under ​http://docs.djangoproject.com/en/dev/ref/contrib/formtools/form-preview/#django.contrib.formtools.FormPreview rather than being spread across that section and "advanced methods".

2. Important methods which explicitly (in the code) are marked as "METHODS SUBCLASSES MIGHT OVERRIDE IF APPROPRIATE" are not documented at all.

3. The crossref targets for the FormPreview class are all wrong. They are currently pointed at django.contrib.formtools.FormPreview when they should be pointed at django.contrib.formtools.preview.FormPreview. I assume this is either a lack of indentation or a lack of a currentmodule directive.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

Moved to ​https://github.com/django/django-formtools/issues/32