iio: Add ABI documentation for RGBC intensity hardware gain factor

This patch adds documentation for RGBC in_intensity_*_hardwaregain.
There is at least one user for these, ADJD-S311-CR999 digital color sensor
driver.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>