hwmon: (w83781d) Additional information about AS99127F PWM

This information was provided in lm-sensors ticket #2350:
http://www.lm-sensors.org/ticket/2350

This is IMHO still not enough to be able to safely implement fan
control support for the AS99127F, but this is valuable information so
I am adding it to the documentation.

Signed-off-by: Jean Delvare <khali@linux-fr.org>