docs: add Doxygen comment for _vocIdx in SGP40 driver

Author: mikeysklar

src/components/i2c/drivers/WipperSnapper_I2C_Driver_SGP40.h

@@ -131,7 +131,7 @@ class WipperSnapper_I2C_Driver_SGP40 : public WipperSnapper_I2C_Driver {
 
   /** Cached latest measurements (no averaging). */
   uint16_t _rawValue = 0;
-  uint16_t _vocIdx = 0;
+  uint16_t _vocIdx = 0; ///< Index for the VOC (volatile organic compounds) sensor reading
 
   /*******************************************************************************/
   /*!