Clarify Formset documentation.

[Ken Whitesell]

I believe the documentation for formsets is either misleading or possibly
incorrect.

From the existing documentation at
​https://docs.djangoproject.com/en/2.2/topics/forms/formsets/

You might want to allow the user to create several articles at once. 
To create a formset out of an ArticleForm you would do:
   
>>> from django.forms import formset_factory
>>> ArticleFormSet = formset_factory(ArticleForm)
   
You now have created a formset named ArticleFormSet.

It's my understanding that *at this point* you have not yet created a formset.
What you have done is create a formset class, from which you can create
instances of formsets.

It's not until you execute this line:

>>> formset = ArticleFormSet()

that you have actually created the formset.

By analogy, consider the detail provided for both Models and Forms.
On the page ​https://docs.djangoproject.com/en/2.2/ref/models/instances/
it is specified that "To create a new instance of a model, instantiate it like
any other Python class:"

Likewise, with Forms, at ​https://docs.djangoproject.com/en/2.2/ref/forms/api/
it states: "To create an unbound Form instance, simply instantiate the class:"

In both these cases, the distinction is made that there's a fundamental
difference between creating the Model and Form classes, and creating the
instances of those classes.

I believe the same distinction should be made with reference to Formsets,
making it explicitly clear that the formset_factory function creates a
class, and the class needs to be instantiated to create the instances to
be used within a page.

[Carlton Gibson]

​PR.

[Carlton Gibson <carlton.gibson@…>]

In 4c762588:

Fixed #30917 -- Clarified formsets topic documentation.

[Carlton Gibson <carlton.gibson@…>]

In 574f2385:

[3.0.x] Fixed #30917 -- Clarified formsets topic documentation.

Backport of 4c762588ff6748a9a9bad111894fd418c43b74a9 from master

[Carlton Gibson <carlton.gibson@…>]

In 9d15f1ea:

[2.2.x] Fixed #30917 -- Clarified formsets topic documentation.

Backport of 4c762588ff6748a9a9bad111894fd418c43b74a9 from master