CAN: Add functions to clear queues and fix multiple bugs

This commits adds the functions can_clear_transmit_queue() and
can_clear_receive_queue(). Closes #2906

The following bug are fixed:

- CAN_IO_UNUSED is now explicitly cast to enum type. Closes #2825
- Fix multiple documentation errors. Closes #2898, Closes #2794
- can_reconfigure_alerts() returns incorrect current_alerts. Closes #3028
- Add missing header file. Closes #3065

examples/peripherals/can/can_network/can_network_master/main/can_network_example_master_main.c

@@ -131,7 +131,8 @@ static void can_receive_task(void *arg)
     vTaskDelete(NULL);
 }
 
-static void can_transmit_task(void *arg) {
+static void can_transmit_task(void *arg)
+{
     while (1) {
         tx_task_action_t action;
         xQueueReceive(tx_task_queue, &action, portMAX_DELAY);
@@ -158,7 +159,8 @@ static void can_transmit_task(void *arg) {
     vTaskDelete(NULL);
 }
 
-void can_control_task(void *arg) {
+static void can_control_task(void *arg)
+{
     xSemaphoreTake(ctrl_task_sem, portMAX_DELAY);
     tx_task_action_t tx_action;
     rx_task_action_t rx_action;