Improve documentation for Graph.V() and Graph.E() on main docs page

I was trying to help someone out on stack overflow recently: http://stackoverflow.com/questions/41638792/dse-graph-get-all-edges-between-two-vertexes. I was on the main TinkerPop3 Docs page looking for some introduction to g.V() that would be good to copy/paste, but couldn't find anything.

It seems like there might be a stepping stone missing between "The Traversal" and "Graph Traversal Steps" - a stepping stone about using V and E to seed/start a Traversal.

The use of IDs should also be discussed there as well.