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

drivers/iio/chemical/sgp40.c

@@ -14,11 +14,16 @@
  * 1) read raw logarithmic resistance value from sensor
  *    --> useful to pass it to the algorithm of the sensor vendor for
  *    measuring deteriorations and improvements of air quality.
+ *    It can be read from the attribute in_resistance_raw.
  *
- * 2) calculate an estimated absolute voc index (0 - 500 index points) for
- *    measuring the air quality.
+ * 2) calculate an estimated absolute voc index (in_concentration_input)
+ *    with 0 - 500 index points) for measuring the air quality.
  *    For this purpose the value of the resistance for which the voc index
- *    will be 250 can be set up using calibbias.
+ *    will be 250 can be set up using in_resistance_calibbias (default 30000).
+ *
+ *    The voc index is calculated as:
+ *      x = (in_resistance_raw - in_resistance_calibbias) * 0.65
+ *      in_concentration_input = 500 / (1 + e^x)
  *
  * Compensation values of relative humidity and temperature can be set up
  * by writing to the out values of temp and humidityrelative.