iio: distance: srf08: add driver ABI documentation

Add sysfs-bus-iio-distance-srf08 for individual attributes of the driver,
especially:
 - sensitivity which the device documentation calls gain for amplifying the
   signal
 - max_range for limiting the maximum distance for expected echos and
   therefore limiting the time waiting for telegrams
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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
+Contact:	linux-iio@vger.kernel.org
+Description:
+                Show or set the maximum range between the sensor and the
+		first object echoed in meters. Default value is 6.020.
+		This setting limits the time the driver is waiting for a
+		echo.
+		Showing the range of available values is represented as the
+		minimum value, the step and the maximum value, all enclosed
+		in square brackets.
+		Example:
+		[0.043 0.043 11.008]