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.