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 interpret "should return the ``value_to_string`` of the parent class" as:
```
def value_to_string(self, obj):
    return super().value_to_string(obj)
```
which is obviously pointless. Yes, linking to the example would be helpful, but I think that text still needs some revision.