Kernel: Add an explanatory comment to the end of _start()

kernel/src/main.cpp

@@ -78,5 +78,10 @@ extern "C" void _start()
 
     Interrupts::enable();
 
-    while (1) halt();
+    while (1)
+        halt(); // As soon as the first timer interrupt arrives, this idle loop is gone, since the main function is not
+                // registered as a task and thus the scheduler will never schedule this again. We still have to do
+                // something while waiting for a timer interrupt to arrive, though. In fact, in most cases, calling
+                // halt() once would be enough, since that function halts the CPU until the next interrupt (most likely
+                // the timer one) arrives. But we have to guarantee this function never returns.
 }