iio:Documentation: Add documentation for label channel attribute

If a label is defined for this channel add that to the channel specific
attributes. This is useful for userspace to be able to identify an
individual channel.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20200928090959.88842-2-cristian.pop@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1715,6 +1715,16 @@ Description:
 
 		Raw counter device counters direction for channel Y.
 
+What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_label
+What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_label
+KernelVersion:	5.8
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Optional symbolic label to a device channel.
+		If a label is defined for this channel add that to the channel
+		specific attributes. This is useful for userspace to be able to
+		better identify an individual channel.
+
 What:		/sys/bus/iio/devices/iio:deviceX/in_phaseY_raw
 KernelVersion:	4.18
 Contact:	linux-iio@vger.kernel.org