docs: hwmon: Add attributes to report rated values

New attributes (rated_min/rated_max) are being added the hwmon ABI.
They are needed to cover rated values documented by PMBus
specification:
PMBus Power System Mgt Protocol Specification - Part II – Revision 1.3.1,
chapters: 22.3.1 - 22.3.10, 22.3.15

Signed-off-by: Zbigniew Lukwinski <zbigniew.lukwinski@linux.intel.com>
Link: https://lore.kernel.org/r/1596224237-32280-2-git-send-email-zbigniew.lukwinski@linux.intel.com


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