## 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` -> `VertexId`

## How was this patch tested?
No tests, only docs is modified

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #15875 from zhengruifeng/update_graphop_doc.

(cherry picked from commit c31def1d)
Signed-off-by: Reynold Xin <rxin@databricks.com>