ADS1115: update docs (PX4#21638)

src/drivers/adc/ads1115/ads1115_main.cpp

@@ -134,6 +134,24 @@ void ADS1115::RunImpl()
 
 void ADS1115::print_usage()
 {
+
+	PRINT_MODULE_DESCRIPTION(
+		R"DESCR_STR(
+### Description
+
+Driver to enable an external [ADS1115](https://www.adafruit.com/product/1085) ADC connected via I2C.
+
+The driver is included by default in firmware for boards that do not have an internal analog to digital converter,
+such as [PilotPi](../flight_controller/raspberry_pi_pilotpi.md) or [CUAV Nora](../flight_controller/cuav_nora.md)
+(search for `CONFIG_DRIVERS_ADC_ADS1115` in board configuration files).
+
+It is enabled/disabled using the
+[ADC_ADS1115_EN](../advanced_config/parameter_reference.md#ADC_ADS1115_EN)
+parameter, and is disabled by default.
+If enabled, internal ADCs are not used.
+
+)DESCR_STR");
+
 	PRINT_MODULE_USAGE_NAME("ads1115", "driver");
 	PRINT_MODULE_USAGE_COMMAND("start");
 	PRINT_MODULE_USAGE_PARAMS_I2C_SPI_DRIVER(true, false);