Now PySpark on YARN with cluster mode is supported so let's update doc.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #6040 from sarutak/update-doc-for-pyspark-on-yarn and squashes the following commits:

ad9f88c [Kousuke Saruta] Brushed up sentences
469fd2e [Kousuke Saruta] Merge branch 'master' of https://github.com/apache/spark into update-doc-for-pyspark-on-yarn
fcfdb92 [Kousuke Saruta] Updated doc for PySpark on YARN with cluster mode