codal_port: Add comments for functions that run on HW IRQs.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

src/codal_port/drv_system.c

@@ -35,7 +35,7 @@ void microbit_system_init(void) {
     accelerometer_up_to_date = false;
 }
 
-// Called every 6ms
+// Called every 6ms on a hardware interrupt.
 // TODO: should only enable this when system is ready
 // TODO: perhaps only schedule the callback when we need it
 void microbit_hal_timer_callback(void) {

src/codal_port/modmusic.c

@@ -76,6 +76,7 @@ STATIC int music_output_period_us(uint32_t period) {
     return microbit_hal_pin_set_analog_period_us(MICROBIT_HAL_PIN_MIXER, period);
 }
 
+// This runs on a hardware interrupt.
 void microbit_music_tick(void) {
     if (music_data == NULL) {
         // music module not yet imported