graph/encoding/dot: Unmarshal docs do not say only one graph may be unmarshaled
Created by: kortschak
The dot.Unmarshal function will only decode a graph when there is a single graph in the DOT description. There should be a warning that this is the case in the doc comment.
As an extension, it might be worthwhile giving the first graph back even if there is more than one, but also returning the non-nil error.
@mewmew Thoughts?