Add docs for new configs & code style

Author: li-zhihui

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -50,8 +50,11 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, actorSystem: A
   val conf = scheduler.sc.conf
   private val timeout = AkkaUtils.askTimeout(conf)
   private val akkaFrameSize = AkkaUtils.maxFrameSizeBytes(conf)
+  // Submit stage only after (registered executors / total executors) arrived the ratio.
   var minRegisteredRatio = conf.getDouble("spark.scheduler.minRegisteredExecutorsRatio", 0)
-  val maxRegisteredWaitingTime = conf.getInt("spark.scheduler.maxRegisteredExecutorsWaitingTime", 30000)
+  // Whatever minRegisteredExecutorsRatio is arrived, submit stage after the time(milliseconds).
+  val maxRegisteredWaitingTime =
+    conf.getInt("spark.scheduler.maxRegisteredExecutorsWaitingTime", 30000)
   val createTime = System.currentTimeMillis()
   var ready = if (minRegisteredRatio <= 0) true else false