fix spark-6033, clarify the spark.worker.cleanup behavior in standalone mode

jira case spark-6033 https://issues.apache.org/jira/browse/SPARK-6033

In standalone deploy mode, the cleanup will only remove the stopped application's directories.

The original description about the cleanup behavior is incorrect.

Author: 许鹏 <peng.xu@fraudmetrix.cn>

Closes #4803 from hseagle/spark-6033 and squashes the following commits:

927a6a0 [许鹏] fix the incorrect description about the spark.worker.cleanup in standalone mode