staging: iio: Documentation: Add missing sysfs docs for angle channel

The iio resolver drivers in staging use angle channels. This patch
add missing documentation for this type of channel.

As was discussed in [1], radians is chosen as the unit, to match the
unit of angular velocity.

[1] https://marc.info/?l=linux-driver-devel&m=152190078308330&w=2

Signed-off-by: David Veenstra <davidjulianveenstra@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -190,6 +190,13 @@ Description:
 		but should match other such assignments on device).
 		Units after application of scale and offset are m/s^2.
 
+What:		/sys/bus/iio/devices/iio:deviceX/in_angl_raw
+KernelVersion:	4.17
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Angle of rotation. Units after application of scale and offset
+		are radians.
+
 What:		/sys/bus/iio/devices/iio:deviceX/in_anglvel_x_raw
 What:		/sys/bus/iio/devices/iio:deviceX/in_anglvel_y_raw
 What:		/sys/bus/iio/devices/iio:deviceX/in_anglvel_z_raw
@@ -297,6 +304,7 @@ 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
+What:		/sys/bus/iio/devices/iio:deviceX/in_angl_offset
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -350,6 +358,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_humidityrelative_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_velocity_sqrt(x^2+y^2+z^2)_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_illuminance_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_countY_scale
+What:		/sys/bus/iio/devices/iio:deviceX/in_angl_scale
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description: