Document form Field's template_name argument

[Tim Schilling]

The change to support form field rendering in Ticket #34077 stopped short of documenting that a field can have a customized template specified on the field itself. This template would be passed to the ​BoundField which we have public documentation for.

This change would mean documenting template_name in the ​Field.__init__ method here.

I suppose one reason to not do this is if this particular API isn't something we want to commit to. I think since it hasn't changed in a few years, it appears stable.

The usage we'd want to make developers aware of is that they can do today:

class ProfileForm(forms.Form):
    image = forms.ImageField(
        template_name="my_forms/fields/image_with_preview.html"
    )