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>