iio: Add ABI documentation for proximity

has been in use for a long time, but never documented
proximity is unit-less, except otherwise stated, see e.g.
the as3935 driver where it is meter
Signed-off-by: NPeter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -895,6 +895,19 @@ Description:
 		on-chip EEPROM. After power-up or chip reset the device will
 		automatically load the saved configuration.
 
+What:		/sys/.../iio:deviceX/in_proximity_raw
+What:		/sys/.../iio:deviceX/in_proximity_input
+What:		/sys/.../iio:deviceX/in_proximityY_raw
+KernelVersion:	3.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Proximity measurement indicating that some
+		object is near the sensor, usually be observing
+		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.
+
 What:		/sys/.../iio:deviceX/in_illuminanceY_input
 What:		/sys/.../iio:deviceX/in_illuminanceY_raw
 What:		/sys/.../iio:deviceX/in_illuminanceY_mean_raw