Update the import path for the FormSet classes and factories in documentation.

[loic84]

Since #21489, FormSet classes and factories are exposed directly on the django.forms package.

Examples in the docs should be updated to reflect this change at some point. The docs update is delayed for a couple of releases to help people who misuse the dev docs with an earlier version of Django so they don't get confusing import errors.

The original ticket (#21489) has a tentative patch and discussions relevant to this ticket.

[Tim Graham]

This change was made in Django 1.7 and all older versions are now unsupported so it seems like a good time to make the change. See 1c7a83ee8e3da431d9d21dae42da8f1f89973f7c for the functions and classes to update.

[Bryan Marty]

This is my first ticket / contribution to the Django project.

I have created a topic branch at:
​https://github.com/bxm156/django/tree/ticket_21516

Commit:
​https://github.com/bxm156/django/commit/9c117b9bd62ea090f1d8b4d7fb7345ebaaf9e836

Ran make html in docs locally, the build succeeded.

[Bryan Marty]

I have updated my changes to reflect timgraham's feedback.

​https://github.com/django/django/pull/5492

[Tim Graham <timograham@…>]

In 455034d4:

Fixed #21516 -- Updated imports paths for some formset functions/classes.

Since refs #21489, FormSet classes and factories are exposed on the
django.forms package.

[Tim Graham <timograham@…>]

In 44f177b5:

[1.9.x] Fixed #21516 -- Updated imports paths for some formset functions/classes.

Since refs #21489, FormSet classes and factories are exposed on the
django.forms package.

Backport of 455034d4df048010de4ae0a9a2392b70d1463c61 from master

[Tim Graham <timograham@…>]

In 42f25467:

[1.8.x] Fixed #21516 -- Updated imports paths for some formset functions/classes.

Since refs #21489, FormSet classes and factories are exposed on the
django.forms package.

Backport of 455034d4df048010de4ae0a9a2392b70d1463c61 from master