Document that DFS/BFS views never return the source

[akrzemi1]

I was surprised to find that DFS/BFS views never return the source vertex.

The example is here.

I suppose there is a reason for it, and it is not a big loss, as I know to include the source vertex, given that I passed it as argument to the view factory. But it is counter to the informal intuition that BFS iterates over all vertices.

This needs to be documented both in the graph-v2 library and in the papers.