iio: Add ABI documentation for in_rot offset value

This patch adds ABI documentation entries for in_rot_offset.
At least one user for these is present that is the  HID Sensors Driver.
Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -254,6 +254,7 @@ 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
+What:		/sys/bus/iio/devices/iio:deviceX/in_rot_offset
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description: