SPARK-1178: missing document of spark.scheduler.revive.interval

https://spark-project.atlassian.net/browse/SPARK-1178

The configuration on spark.scheduler.revive.interval is undocumented but actually used

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala#L64

Author: CodingCat <zhunansjtu@gmail.com>

Closes #74 from CodingCat/SPARK-1178 and squashes the following commits:

783ec69 [CodingCat] missing document of spark.scheduler.revive.interval