Addresses ticket 22809

This still requires a revision, but gives an idea of what I've been doing for the ticket.

28998b0 only moves the .. class:: Field declaration.
387370d removes a similar statement already appearing in the document
86aa1f0 does a major formatting change on the document, but only changing "\n" and " " characters.
745eec2 fixes [ticket 22809](https://code.djangoproject.com/ticket/22809), moving all method declarations from howto to refs, changing the text the least possible amount to mantain compatibility.

Both sections of the docs still require a major editorial process on wording and references to models.
I think this section would be better at the bottom of the page because it's not relevant for most users who typically start off by writing custom fields.