spark git commit: [SPARK-18428][DOC] Update docs for GraphX

Repository: spark Updated Branches: refs/heads/branch-2.1 27999b366 -> 649c15fae [SPARK-18428][DOC] Update docs for GraphX ## What changes were proposed in this pull request? 1, Add link of `VertexRDD` and `EdgeRDD` 2, Notify in `Vertex and Edge RDDs` that not all methods are listed 3,

spark git commit: [SPARK-18428][DOC] Update docs for GraphX

Repository: spark Updated Branches: refs/heads/master c07187823 -> c31def1dd [SPARK-18428][DOC] Update docs for GraphX ## What changes were proposed in this pull request? 1, Add link of `VertexRDD` and `EdgeRDD` 2, Notify in `Vertex and Edge RDDs` that not all methods are listed 3, `VertexID`