Fixed #13184 -- Document the requirement that custom model fields using...

This ensures that the :meth:`to_python` method, documented below, will always be

called when the attribute is initialized.

ModelForms and custom fields

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you use :class:`~django.db.models.SubfieldBase`, :meth:`to_python`

will be called every time an instance of the field is assigned a

value. This means that whenever a value may be assigned to the field,

you need to ensure that it will be of the correct datatype, or that

you handle any exceptions.

This is especially important if you use :ref:`ModelForms

<topics-forms-modelforms>`. When saving a ModelForm, Django will use

form values to instantiate model instances. However, if the cleaned

form data can't be used as valid input to the field, the normal form

validation process will break.

Therefore, you must ensure that the form field used to represent your

custom field performs whatever input validation and data cleaning is

necessary to convert user-provided form input into a

`to_python()`-compatible model field value. This may require writing a

custom form field, and/or implementing the :meth:`formfield` method on

your field to return a form field class whose `to_python()` returns the

correct datatype.

Documenting your Custom Field

-----------------------------