update comment

core/src/main/java/com/taobao/arthas/core/command/monitor200/TimeTunnelCommand.java

@@ -52,6 +52,7 @@ import static java.lang.String.format;
         Constants.WIKI + Constants.WIKI_HOME + "tt")
 public class TimeTunnelCommand extends EnhancerCommand {
     // 时间隧道(时间碎片的集合)
+    // TODO 并非线程安全？
     private static final Map<Integer, TimeFragment> timeFragmentMap = new LinkedHashMap<Integer, TimeFragment>();
     // 时间碎片序列生成器
     private static final AtomicInteger sequence = new AtomicInteger(1000);