[INFINITY-3127] Cleanup HDFS service settings docs (#2304)
[INFINITY-3127] Cleanup HDFS service settings docs (#2304)

* Cleanup HDFS service settings docs

* Address review comments w.r.t. heap size.

mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons