boards/stm32l476g-disco: add doxygen doc to ADC config

boards/stm32l476g-disco/include/periph_conf.h

@@ -80,6 +80,35 @@ static const uart_conf_t uart_config[] = {
 #define UART_NUMOF          ARRAY_SIZE(uart_config)
 /** @} */
 
+/**
+ * @brief    ADC configuration
+ *
+ * Note that we do not configure all ADC channels,
+ * and not in the STM32L476VG order. Instead, we
+ * just define 5 ADC channels, for the next adjacent
+ * 5 pins, from 10 to 14, in the header P1 and the
+ * internal VBAT channel.
+ *
+ * To find appropriate device and channel find in the
+ * board manual, table showing pin assignments and
+ * information about ADC - a text similar to ADC[X]_IN[Y],
+ * where:
+ * [X] - describes used device - indexed from 0,
+ * for example ADC12_IN10 is device 0 or device 1,
+ * [Y] - describes used channel - indexed from 1,
+ * for example ADC12_IN10 is channel 10
+ *
+ * For STM32L476VG this information is in MCU datasheet,
+ * Table 16, page 73.
+ *
+ * VBAT is connected ADC1_IN18 or ADC3_IN18 and a voltage divider
+ * is used, so that only 1/3 of the actual VBAT is measured. This
+ * allows for a supply voltage higher than the reference voltage.
+ *
+ * For STM32L476VG more information is provided in MCU datasheet,
+ * in section 3.15.3 - Vbat battery voltage monitoring, page 42.
+ * @{
+ */
 static const adc_conf_t adc_config[] = {
     {GPIO_PIN(PORT_A, 0), 0, 5},  /*< ADC12_IN5  */
     {GPIO_PIN(PORT_A, 5), 0, 10}, /*< ADC12_IN10 */
@@ -89,9 +118,16 @@ static const adc_conf_t adc_config[] = {
     {GPIO_UNDEF, 0, 18}, /* VBAT */
 };
 
-#define VBAT_ADC            ADC_LINE(5) /**< VBAT ADC line */
+/**
+ * @brief VBAT ADC line
+ */
+#define VBAT_ADC            ADC_LINE(5)
 
+/**
+ * @brief Number of ADC devices
+ */
 #define ADC_NUMOF           ARRAY_SIZE(adc_config)
+/** @} */
 
 #ifdef __cplusplus
 }