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

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.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>