ABI: hwmon: Document "label" sysfs attribute

Add the "label" sysfs attribute, which can contain a descriptive label
that allows to uniquely identify a device within the system.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220110182256.30763-2-paul@crapouillou.net


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