[SPARK-9570] [DOCS] Consistent recommendation for submitting spark apps to...

[SPARK-9570] [DOCS] Consistent recommendation for submitting spark apps to YARN, -master yarn --deploy-mode x vs -master yarn-x'.

Recommend `--master yarn --deploy-mode {cluster,client}` consistently in docs.
Follow-on to https://github.com/apache/spark/pull/8385
CC nssalian

Author: Sean Owen <sowen@cloudera.com>

Closes #8968 from srowen/SPARK-9570.