improved documentation

Author: jkbradley

graphx/src/main/scala/org/apache/spark/graphx/Graph.scala

@@ -105,12 +105,14 @@ abstract class Graph[VD: ClassTag, ED: ClassTag] protected () extends Serializab
   def checkpoint(): Unit
 
   /**
-   * Return whether this Graph has been checkpointed or not
+   * Return whether this Graph has been checkpointed or not.
+   * This returns true iff both the vertices RDD and edges RDD have been checkpointed.
    */
   def isCheckpointed: Boolean
 
   /**
-   * Gets the name of the files to which this Graph was checkpointed
+   * Gets the name of the files to which this Graph was checkpointed.
+   * (The vertices RDD and edges RDD are checkpointed separately.)
    */
   def getCheckpointFiles: Seq[String]