hwmon: (tmp421) update documentation

Sysfs interface of the tmp421 driver was extended in the recent patches
so lets update the documentation to reflect that.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/6ae22fdf18513ddd09f430ef9c5e46c17e27886b.1634206677.git.krzysztof.adamski@nokia.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>