[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 <brennon.york@capitalone.com>

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'