"Working with forms" docs need improving

[Daniele Procida]

Our main "introduction to forms" documentation is ​https://docs.djangoproject.com/en/dev/topics/forms/

It lacks sufficient basic explanation of web form handling in general to be useful to someone who is tackling this for the first time.

It also contains some sections that mention, but offer no explanation of, matters that don't need to be in an introductory document.

It also interrupts its own basic explanations with unnecessary reference to much more advanced concepts.

[Daniele Procida]

Patch in ​https://github.com/django/django/pull/2776.

[Tim Graham]

Some more comments have been left on the PR.

[Tim Graham <timograham@…>]

In 60375eda324d708870dc2ba18b4ac4d6114a68f6:

[1.7.x] Fixed #22796 -- Added a more basic explanations of forms.

Thanks bmispelon, kezabelle, jorgecarleitao, and timgraham for reviews.

Backport of cd20b5d186 from master