HDFS-16162.Improve DFSUtil#checkProtectedDescendants() related parameter comments. (#3295)

Co-authored-by: zhujianghua <zhujianghua@zhujianghuadeMacBook-Pro.local>
Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>
Reviewed-by: Viraj Jasani <vjasani@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSUtil.java

@@ -1835,6 +1835,7 @@ public class DFSUtil {
    * Throw if the given directory has any non-empty protected descendants
    * (including itself).
    *
+   * @param fsd the namespace tree.
    * @param iip directory whose descendants are to be checked.
    * @throws AccessControlException if a non-empty protected descendant
    *                                was found.