iio: ABI: Document in_concentration_co2_scale

Document the 'in_concentration_co2_scale' standard IIO attribute.
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Link: https://lore.kernel.org/r/20210920135413.140310-5-jacopo+renesas@jmondi.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -429,6 +429,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_angl_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_x_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_y_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_z_scale
+What:		/sys/bus/iio/devices/iio:deviceX/in_concentration_co2_scale
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description: