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

[brennonyork]

Turns out, per the convo on the JIRA, 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.

[SparkQA]

Test build #28569 has started for PR 5015 at commit 92288f7.

• This patch merges cleanly.

[brennonyork]

ok to test

[brennonyork]

/cc @ankurdave

[SparkQA]

Test build #28570 has started for PR 5015 at commit 1e1d1e5.

• This patch merges cleanly.

[SparkQA]

Test build #28570 has finished for PR 5015 at commit 1e1d1e5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #28569 has finished for PR 5015 at commit 92288f7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28569/
Test PASSed.

[brennonyork]

/cc @srowen too as you this is a, seemingly trivial, doc change

[srowen]

LGTM, straightforward doc update.