iio: ABI: document in_illuminance_hardwaregain

This attribute is used for the vl6180 (see vl6180.c), but it is still
not documented. Add it to the _hardwaregain list.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241215-iio_abi_in_illuminance_hardwaregain-v1-1-d94a59efb937@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>