Documentation: hwmon: Add descriptions for ina2xx sysfs entries

There are a few sysfs entries being exposed to user space by the
ina2xx hwmon driver while not getting explicitly documented. So
this patch just adds a description section for them.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>