`Form.clean()` is different from the other methods listed here in that it's just a hook that you have to implement yourself.  I suggest emphasizing that with something like the following: "Implement a ``clean()`` method on your ``Form`` when you must add custom validation for fields that are interdependent. See :ref:`cleaning-and-validating-fields-that-depend-on-each-other` for example usage."

I think it'll be best to leave it that, rather than repeat examples that come from the other section.