Add example of modelformset_factory form argument to document

[alasdair]

There are not any examples in the documentation of using the form argument of ​modelformset_factory.

Often, it is not required, because you can specify arguments like fields, excludes and widgets, for which there are examples.

However, if you want to write any custom validation, then it is necessary to specify form, and I think it would be good to provide an example.

[alasdair]

Pull request ​https://github.com/django/django/pull/2012

[Tim Graham <timograham@…>]

In 1fa681ee11ac234cdc515338397625f514e85b73:

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

[Tim Graham <timograham@…>]

In e4174da34ab239f04d9c79fec9822b01d63e453b:

[1.5.x] Fixed #21539 -- Added example of modelformset_factory's form argument

Backport of 1fa681ee11 from master

[Tim Graham <timograham@…>]

In 15ab4936d7d7c351ab8eea5f8c2765632429c9ed:

[1.6.x] Fixed #21539 -- Added example of modelformset_factory's form argument

Backport of 1fa681ee11 from master