Types in documentation

[mtorpey]

The doc is coming along nicely, but there are some improvements that could be made.

1. The "Returns" statement should just describe the type of the thing that's returned. So for example, GraphvizHead returns "the head of the provided graphviz edge" but should instead return "A graphviz edge object". AsString returns not "the dot representation of the graphviz object" but simply "a string."
2. Specify the types of the arguments in the doc. I believe Autodoc has a construct for this.