iio: sgp40: retain documentation in driver

Retain documentation on how the voc index is actually calculated in
driver code as it'll be removed in Documentation.

This is a follow up on patch "[PATCH] iio: ABI: remove duplicate
in_resistance_calibbias" from David.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Link: https://patch.msgid.link/ZsWdFOIkDtEB9WGO@mail.your-server.de


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