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 agree the text must be improved here.

However, the problem persists in the howto:
> "Calling ``Field._get_val_from_obj(obj)`` is the best way to get the value to serialize."

Do we want to expose a private method (``_``) like this?

cc: @akaariai