iio: Add ABI documentation for offset and scaled humidity

This patch adds ABI documentation entries for in_humidityrelative_offset
and in_humidityrelative_scale, since there is at least one user for these,
Si7005 humidity and temperature sensor driver.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>