HDFS-13892. Disk Balancer: Make execute command documentation better.

Contributed by  Ranith Sardar.

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HDFSDiskbalancer.md

@@ -78,7 +78,9 @@ Execute command takes a plan command executes it against the datanode that plan
 `hdfs diskbalancer -execute /system/diskbalancer/nodename.plan.json`
 
 This executes the plan by reading datanode’s address from the plan file.
-
+When DiskBalancer executes the plan, it is the beginning of an asynchronous process that can take a long time.
+So, query command can help to get the current status of execute command.
+ 
 | COMMAND\_OPTION    | Description |
 |:---- |:---- |
 | `-skipDateCheck` |  Skip date check and force execute the plan.|