make assumption regarding edges over multiple layers

doc/chapter/2architecture.tex

@@ -5,6 +5,7 @@ The following assumptions are made for the implementation of the node placement
     \item There are no port constraints.
     \item There are no labels.
     \item There are no cross-hierarchy edges.
+    \item No edges over multiple layers (the previous phases should have added dummy nodes).
 \end{itemize}
 
 \section{Input File Format}\label{sec:inputFileFormat}