HDFS-12078. Add time unit to the description of property dfs.namenode.stale.datanode.interval in hdfs-default.xml. Contributed by Weiwei Yang.

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -1811,8 +1811,8 @@
   <name>dfs.namenode.stale.datanode.interval</name>
   <value>30000</value>
   <description>
-    Default time interval for marking a datanode as "stale", i.e., if 
-    the namenode has not received heartbeat msg from a datanode for 
+    Default time interval in milliseconds for marking a datanode as "stale",
+    i.e., if the namenode has not received heartbeat msg from a datanode for
     more than this time interval, the datanode will be marked and treated 
     as "stale" by default. The stale interval cannot be too small since 
     otherwise this may cause too frequent change of stale states.