Docs: add function description to dac_out_test

Refs: #8

src/main/esp32_eurorack_generic.c

@@ -67,6 +67,7 @@ static esp_err_t dac_ack_test(void)
   return ret;
 }
 
+/* Brings all DAC output channels from 0 -> ~+5V */
 static esp_err_t dac_out_test(void)
 {
   esp_err_t ret;