iio:Documentation: Add documentation for hinge sensor channels

Add channel description for hinge sensor, including channel label
attribute and raw data description.
Signed-off-by: Ye Xiang <xiang.ye@intel.com>
Link: https://lore.kernel.org/r/20201215054444.9324-4-xiang.ye@intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -198,6 +198,7 @@ Description:
 		Units after application of scale and offset are m/s^2.
 
 What:		/sys/bus/iio/devices/iio:deviceX/in_angl_raw
+What:		/sys/bus/iio/devices/iio:deviceX/in_anglY_raw
 KernelVersion:	4.17
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -1812,3 +1813,13 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Unscaled light intensity according to CIE 1931/DIN 5033 color space.
 		Units after application of scale are nano nanowatts per square meter.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_anglY_label
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Optional symbolic label for channel Y.
+		For Intel hid hinge sensor, the label values are:
+		hinge, keyboard, screen. It means the three channels
+		each correspond respectively to hinge angle, keyboard angle,
+		and screen angle.