[SPARK-4600][GraphX]: org.apache.spark.graphx.VertexRDD.diff does not work

Turns out, per the [convo on the JIRA](https://issues.apache.org/jira/browse/SPARK-4600), `diff` is acting exactly as should. It became a large misconception as I thought it meant set difference, when in fact it does not. To that extent I merely updated the `diff` documentation to, hopefully, better reflect its true intentions moving forward.

Author: Brennon York <[email protected]>

Closes #5015 from brennonyork/SPARK-4600 and squashes the following commits:

1e1d1e5 [Brennon York] reverted internal diff docs
92288f7 [Brennon York] reverted both the test suite and the diff function back to its origin functionality
f428623 [Brennon York] updated diff documentation to better represent its function
cc16d65 [Brennon York] Merge remote-tracking branch 'upstream/master' into SPARK-4600
66818b9 [Brennon York] added small secondary diff test
99ad412 [Brennon York] Merge remote-tracking branch 'upstream/master' into SPARK-4600
74b8c95 [Brennon York] corrected  method by leveraging bitmask operations to correctly return only the portions of  that are different from the calling VertexRDD
9717120 [Brennon York] updated diff impl to cause fewer objects to be created
710a21c [Brennon York] working diff given test case
aa57f83 [Brennon York] updated to set ShortestPaths to run 'forward' rather than 'backward'

Author: Brennon York

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

@@ -122,8 +122,11 @@ abstract class VertexRDD[VD](
   def mapValues[VD2: ClassTag](f: (VertexId, VD) => VD2): VertexRDD[VD2]
 
   /**
-   * Hides vertices that are the same between `this` and `other`; for vertices that are different,
-   * keeps the values from `other`.
+   * For each vertex present in both `this` and `other`, `diff` returns only those vertices with
+   * differing values; for values that are different, keeps the values from `other`. This is
+   * only guaranteed to work if the VertexRDDs share a common ancestor.
+   *
+   * @param other the other VertexRDD with which to diff against.
    */
   def diff(other: VertexRDD[VD]): VertexRDD[VD]