engine: graph: Improve comments

Clarify that we're referring to cycles in the graph, since it needs to
be a DAG.

engine/graph/autoedge/autoedge.go

@@ -90,8 +90,8 @@ func AutoEdge(graph *pgraph.Graph, debug bool, logf func(format string, v ...int
 		}
 	}
 
-	// It would be great to ensure we didn't add any loops here, but instead
-	// of checking now, we'll move the check into the main loop.
+	// It would be great to ensure we didn't add any graph cycles here, but
+	// instead of checking now, we'll move the check into the main loop.
 	return nil
 }