Improve scheduler delay tooltip.

As a result of shivaram's experience debugging long scheduler delay, I think we should improve the tooltip to point people in the right direction if scheduler delay is large.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #1488 from kayousterhout/better_tooltips and squashes the following commits:

22176fd [Kay Ousterhout] Improve scheduler delay tooltip.

core/src/main/scala/org/apache/spark/ui/ToolTips.scala

@@ -20,9 +20,9 @@ package org.apache.spark.ui
 private[spark] object ToolTips {
   val SCHEDULER_DELAY =
     """Scheduler delay includes time to ship the task from the scheduler to
-       the executor, and time the time to send a message from the executor to the scheduler stating
-       that the task has completed. When the scheduler becomes overloaded, task completion messages
-       become queued up, and scheduler delay increases."""
+       the executor, and time to send the task result from the executor to the scheduler. If
+       scheduler delay is large, consider decreasing the size of tasks or decreasing the size
+       of task results."""
 
   val INPUT = "Bytes read from Hadoop or from Spark storage."