for #3666 for delete lmq topic comment

Author: tianliuliu

store/src/main/java/org/apache/rocketmq/store/MessageStore.java

@@ -400,8 +400,11 @@ QueryMessageResult queryMessage(final String topic, final String key, final int
 
     /**
      * Clean unused lmq topic.
-     *
-     * @param topic valid topic
+     * When calling to clean up the lmq topic,
+     * the lmq topic cannot be used to write messages at the same time,
+     * otherwise the messages of the cleaning lmq topic may be lost,
+     * please call this method with caution
+     * @param topic lmq topic
      */
     void cleanUnusedLmqTopic(String topic);
 }