iio: Add ABI documentation for scaled voltage

This patch adds an entry in ABI documentation for in_voltage-voltage_scale.
It has at least one user driver, adis16220, in accel driver.
Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -259,6 +259,7 @@ Description:
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_supply_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltage_scale
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltage-voltage_scale
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_scale
 What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_scale