This fixes what appear to be some code errors/typos:
* The `FormMutation` class was renamed to `DjangoFormMutation`, and `ModelFormMutation` to `DjangoModelFormMutation`, in 40610c64a3.
* `form_valid` was renamed to `perform_mutate` in 463ce68b16.
It also clarifies a few things that I found confusing:
* It explicitly mentions that `perform_mutate` is a class method.
* The code samples now import the form classes from their packages, so readers know where to import them from too.