Complete hdfs-rbf-default.xml

Author: hfutatzhanghb

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

@@ -60,7 +60,8 @@
     <name>dfs.federation.router.async.rpc.ns.handler.count</name>
     <value>nsPlaceholder1:0,nsPlaceholder2:0</value>
     <description>
-
+      The number of asynchronous handlers per nameservice, separated by commas, internally separated by colons.
+      The identifier of nameservice is in dfs.nameservices configuration entry.
     </description>
   </property>