hwmon: Update documentation on fan_max

Add fan_max description.

Add fan limit alarm 'max_alarm' to the alarm section.

Signed-off-by: Christian Engelmayer <christian.engelmayer@frequentis.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>