iio: Add ABI documentation for in_magn offset value

This patch adds ABI documentation entries for in_magn_offset.
There is one user, hid-sensor-magn-3d in drivers/iio/magnetometer.
Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -253,6 +253,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_temp_offset
 What:		/sys/bus/iio/devices/iio:deviceX/in_pressureY_offset
 What:		/sys/bus/iio/devices/iio:deviceX/in_pressure_offset
 What:		/sys/bus/iio/devices/iio:deviceX/in_humidityrelative_offset
+What:		/sys/bus/iio/devices/iio:deviceX/in_magn_offset
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description: