explain more about priority donation on unlock

src/rt.c

@@ -630,7 +630,8 @@ static void rt_syscall_exec(struct rt_syscall_record *record)
         rt_atomic_store(&mutex->holder, 0, RT_ATOMIC_RELEASE);
         rt_list_remove(&mutex->list);
         /* When unlocking, the only donated priority that can change is the
-         * unlocking task's. */
+         * unlocking task's, because the task isn't blocked waiting on any
+         * other mutex, otherwise it wouldn't be running. */
         task_donate(rt_active_task);
         wake_mutex_waiter(mutex);
         break;