Better non-normative text for empty function calls.

The previous text contradicted the statements about functions with no outputs
in the pure chapter.

chapters/functions.tex

@@ -913,8 +913,8 @@ \subsection{Empty Function Calls}\label{empty-function-calls}
 An \emph{empty} function call is a call that does not return any results.
 
 \begin{nonnormative}
-An empty call is of limited use in Modelica since a function call without results does not contribute to the simulation, and is not
-allowed to have side-effects that influence the simulation state.
+An empty call is of limited use in Modelica since a function call without results does not contribute to the simulation,
+but it is useful to check assertions and in certain cases for desired side-effects, see \cref{pure-modelica-functions}.
 \end{nonnormative}
 
 An empty call can occur either as a kind of ``null equation'' or ``null statement''.