[SPARK-18495][UI] Document meaning of green dot in DAG visualization

## What changes were proposed in this pull request?

A green dot in the DAG visualization apparently means that the referenced RDD is cached. This is not documented anywhere except in this [blog post](https://databricks.com/blog/2015/06/22/understanding-your-spark-application-through-visualization.html).
It would be good if the Web UI itself documented this somehow (perhaps in the tooltip?) so that the user can naturally learn what it means while using the Web UI.

before pr:

![dingtalk20170125180158](https://cloud.githubusercontent.com/assets/7402327/22286167/37910ea2-e329-11e6-9aae-03dce6fceee2.png)
![dingtalk20170125180218](https://cloud.githubusercontent.com/assets/7402327/22286178/421f3132-e329-11e6-8283-ba6bbd15cfb0.png)

after pr:
![dingtalk20170125175704](https://cloud.githubusercontent.com/assets/7402327/22286216/56a0050a-e329-11e6-813d-2be96b5ab7f1.png)
![dingtalk20170125175749](https://cloud.githubusercontent.com/assets/7402327/22286233/618cd646-e329-11e6-9ce5-10322b169dcb.png)

## How was this patch tested?

Author: uncleGen <hustyugm@gmail.com>

Closes #16702 from uncleGen/SPARK-18495.

core/src/main/scala/org/apache/spark/ui/scope/RDDOperationGraph.scala

@@ -222,7 +222,12 @@ private[ui] object RDDOperationGraph extends Logging {
 
   /** Return the dot representation of a node in an RDDOperationGraph. */
   private def makeDotNode(node: RDDOperationNode): String = {
-    val label = s"${node.name} [${node.id}]\n${node.callsite}"
+    val isCached = if (node.cached) {
+      " [Cached]"
+    } else {
+      ""
+    }
+    val label = s"${node.name} [${node.id}]$isCached\n${node.callsite}"
     s"""${node.id} [label="${StringEscapeUtils.escapeJava(label)}"]"""
   }