Irqmgr + scheduler Docs (#1527)

* irqmgr docs

* Sched_FlushTaskQueue

* scheduler docs

src/code/z_nulltask.c

@@ -0,0 +1,30 @@
+#include "scheduler.h"
+
+#include "macros.h"
+#include "main.h"
+
+/**
+ * Blocks the current thread until all queued scheduler tasks have completed.
+ */
+void Sched_FlushTaskQueue(void) {
+    OSScTask task;
+    OSMesgQueue queue;
+    OSMesg msg[1];
+
+    // Prepare a "NULL" task
+    task.next = NULL;
+    task.flags = OS_SC_NEEDS_RDP | OS_SC_NEEDS_RSP;
+    task.msgQ = &queue;
+    task.msg = NULL;
+    task.framebuffer = NULL;
+    task.list.t.type = M_NULTASK;
+    osCreateMesgQueue(task.msgQ, msg, ARRAY_COUNT(msg));
+
+    // Send it to and wake up the scheduler
+    osSendMesg(&gScheduler.cmdQueue, (OSMesg)&task, OS_MESG_BLOCK);
+    Sched_SendNotifyMsg(&gScheduler);
+
+    // Wait until the task has been processed, indicating that no task is
+    // running and the task queue is now empty.
+    osRecvMesg(&queue, NULL, OS_MESG_BLOCK);
+}