Document, and possibly improve, how to list existing indexes

I wasn't able to find how to list existing indexes (or indices) in the docs.

The command is currently mgmt.getGraphIndexes(Vertex.class) although I'm not sure if vertex-centric would show up under mgmt.getGraphIndexes(Edge.class) instead.

In addition to documentation I think it would be great if we created a getGraphIndexes() method that took no parameters and gave a list of all the indexes and their current state.