SPARK-4770. [DOC] [YARN] spark.scheduler.minRegisteredResourcesRatio doc...

...umented default is incorrect for YARN

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3624 from sryza/sandy-spark-4770 and squashes the following commits:

bd81a3a [Sandy Ryza] SPARK-4770. [DOC] [YARN] spark.scheduler.minRegisteredResourcesRatio documented default is incorrect for YARN