Add some text to example on derivative 'order'

chapters/functions.tex

@@ -1208,6 +1208,7 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
 \end{nonnormative}
 
 \begin{example}
+Use of \lstinline!order! to specify a second order derivative:
 \begin{lstlisting}[language=modelica]
 function foo0 annotation(derivative=foo1);
 end foo0;