Document Master URL format in high availability set up

## What changes were proposed in this pull request?

Add documentation for adding master url in multi host, port format for standalone cluster with high availability with zookeeper.
Referring documentation [Standby Masters with ZooKeeper](http://spark.apache.org/docs/latest/spark-standalone.html#standby-masters-with-zookeeper)

## How was this patch tested?

Documenting the functionality already present.

Author: MirrorZ <chandrika3437@gmail.com>

Closes #17584 from MirrorZ/master.

docs/submitting-applications.md

@@ -143,6 +143,9 @@ The master URL passed to Spark can be in one of the following formats:
 <tr><td> <code>spark://HOST:PORT</code> </td><td> Connect to the given <a href="spark-standalone.html">Spark standalone
         cluster</a> master. The port must be whichever one your master is configured to use, which is 7077 by default.
 </td></tr>
+<tr><td> <code>spark://HOST1:PORT1,HOST2:PORT2</code> </td><td> Connect to the given <a href="spark-standalone.html#standby-masters-with-zookeeper">Spark standalone
+        cluster with standby masters with Zookeeper</a>. The list must have all the master hosts in the high availability cluster set up with Zookeeper. The port must be whichever each master is configured to use, which is 7077 by default.
+</td></tr>
 <tr><td> <code>mesos://HOST:PORT</code> </td><td> Connect to the given <a href="running-on-mesos.html">Mesos</a> cluster.
         The port must be whichever one your is configured to use, which is 5050 by default.
         Or, for a Mesos cluster using ZooKeeper, use <code>mesos://zk://...</code>.