Fixed #21539 -- Added example of modelformset_factory's form argument

Initial attempt at adding an example of the modelformset_factory's form argument. Feedback welcome.

I tried to write the example so that it could easily be backported to 1.5.X and 1.6.X, for example I don't refer to the new widgets or localized_fields options in 1.6.

I couldn't think of a good example to go in the clean method. The only inspiration I got from the current docs was to ensure that the name starts with 'O'.