staging: iio: isl29018: add documentation about the infrared suppression

Add documentation from the ISL29018 Data Sheet (FN6619.4, Oct 8, 2012)
about the infrared suppression that can be controlled
with the proximity_on_chip_ambient_infrared_suppression sysfs attribute.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>