def _add_form_fields(obj, lines):
    """Improve the documentation of a Django Form class.

    This highlights the available fields in the form.
    """
    lines.append('**Form fields:**')
    lines.append('')
    for name, field in obj.base_fields.items():
        field_type = '{}.{}'.format(field.__class__.__module__, field.
            __class__.__name__)
        tpl = '* ``{name}``: {label} (:class:`~{field_type}`)'
        lines.append(tpl.format(name=name, field=field, label=field.label or
            name.replace('_', ' ').title(), field_type=field_type))
