Document spark.shuffle.service.{enabled,port}

CC sryza tgravescs
This is pretty minimal; is there more to say here about the service?

Author: Sean Owen <sowen@cloudera.com>

Closes #7991 from srowen/SPARK-9641 and squashes the following commits:

3bb946e [Sean Owen] Add link to docs for setup and config of external shuffle service
2302e01 [Sean Owen] Document spark.shuffle.service.{enabled,port}