iio:ABI docs: Combine the two instances of docs for sensor_sensitivity

This control on the gain of a measurement used for time of flight sensing
is standard but the expected values for different enviroments may not be.
As we cannot have the same ABI element documented in two files, add a
generic version to sysfs-bus-iio-proximity and a note on the expected
value vs measuring environment for the as3935.

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/sensor_sensitivity is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-distance-srf08:0  ./Documentation/ABI/testing/sysfs-bus-iio-proximity-as3935:8

Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20210117153816.696693-7-jic23@kernel.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-distance-srf08

-What:		/sys/bus/iio/devices/iio:deviceX/sensor_sensitivity
-Date:		January 2017
-KernelVersion:	4.11
-Contact:	linux-iio@vger.kernel.org
-Description:
-		Show or set the gain boost of the amp, from 0-31 range.
-		default 31
-
 What:		/sys/bus/iio/devices/iio:deviceX/sensor_max_range
 Date:		January 2017
 KernelVersion:	4.11

Documentation/ABI/testing/sysfs-bus-iio-proximity

@@ -8,3 +8,17 @@ Description:
 		considered close to the device. If the value read from the
 		sensor is above or equal to the value in this file an object
 		should typically be considered near.
+
+What:		/sys/bus/iio/devices/iio:deviceX/sensor_sensitivity
+Date:		March 2014
+KernelVersion:	3.15
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Proximity sensors sometimes have a controllable amplifier
+		on the signal from which time of flight measurements are
+		taken.
+		The appropriate values to take is dependent on both the
+		sensor and it's operating environment:
+		* as3935 (0-31 range)
+		18 = indoors (default)
+		14 = outdoors

Documentation/ABI/testing/sysfs-bus-iio-proximity-as3935

@@ -6,15 +6,6 @@ Description:
 		Get the current distance in meters of storm (1km steps)
 		1000-40000 = distance in meters
 
-What:		/sys/bus/iio/devices/iio:deviceX/sensor_sensitivity
-Date:		March 2014
-KernelVersion:	3.15
-Contact:	Matt Ranostay <matt.ranostay@konsulko.com>
-Description:
-		Show or set the gain boost of the amp, from 0-31 range.
-		18 = indoors (default)
-		14 = outdoors
-
 What		/sys/bus/iio/devices/iio:deviceX/noise_level_tripped
 Date:		May 2017
 KernelVersion:	4.13