Fixed #23354 -- Documented that ModelChoiceField to_field_name should be a unique field.

    .. attribute:: to_field_name

        This optional argument is used to specify the field to use as the value

        of the choices in the field's widget. By default it is set to ``None``,

        in which case the primary key of each object will be used. For example::

        of the choices in the field's widget. Be sure it's a unique field for

        the model, otherwise the selected value could match more than one

        object. By default it is set to ``None``, in which case the primary key

        of each object will be used. For example::

            # No custom to_field_name

            field1 = forms.ModelChoiceField(queryset=...)