vm: more commentary on lock_console() stuff

vm/debug.cpp

@@ -459,6 +459,9 @@ void factor_vm::factorbug()
 
 	std::cout << "Starting low level debugger..." << std::endl;
 
+	// Even though we've stopped the VM, the stdin_loop thread (see os-*.cpp)
+	// that pumps the console is still running concurrently. We lock a mutex so
+	// the thread will take a break and give us exclusive access to stdin.
 	lock_console();
 
 	if (!fep_help_was_shown) {

vm/os-unix.cpp

@@ -466,6 +466,9 @@ void factor_vm::open_console()
 
 void factor_vm::lock_console()
 {
+	// Lock the stdin_mutex and send the stdin_loop thread a signal to interrupt
+	// any read() it has in progress. When the stdin loop iterates again, it will
+	// try to lock the same mutex and wait until unlock_console() is called.
 	pthread_mutex_lock(&stdin_mutex);
 	pthread_kill(stdin_thread, SIGUSR2);
 }