Add a warning comment above the place where we set srv_main_thread_op_info

to "waiting for server activity" that mentions that this string should not
be changed.

srv/srv0srv.c

@@ -2587,6 +2587,10 @@ suspend_thread:
 
 	mutex_exit(&kernel_mutex);
 
+	/* DO NOT CHANGE THIS STRING. innobase_start_or_create_for_mysql()
+	waits for database activity to die down when converting < 4.1.x
+	databases, and relies on this string being exactly as it is. InnoDB
+	manual also mentions this string in several places. */
 	srv_main_thread_op_info = "waiting for server activity";
 
 	os_event_wait(event);