iio: ABI: add clarification for proximity

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1068,7 +1068,9 @@ Description:
 		reflectivity of infrared or ultrasound emitted.
 		Often these sensors are unit less and as such conversion
 		to SI units is not possible.  Where it is, the units should
-		be meters.
+		be meters.  If such a conversion is not possible, the reported
+		values should behave in the same way as a distance, i.e. lower
+		values indicate something is closer to the sensor.
 
 What:		/sys/.../iio:deviceX/in_illuminanceY_input
 What:		/sys/.../iio:deviceX/in_illuminanceY_raw