[SPARK-21553][SPARK SHELL] Add the description of the default value of master parameter in the spark-shell

When I type spark-shell --help, I find that the default value description for the master parameter is missing. The user does not know what the default value is when the master parameter is not included, so we need to add the master parameter default description to the help information.
[https://issues.apache.org/jira/browse/SPARK-21553](https://issues.apache.org/jira/browse/SPARK-21553)

Author: davidxdh <xu.donghui@zte.com.cn>
Author: Donghui Xu <xu.donghui@zte.com.cn>

Closes #18755 from davidxdh/dev_0728.