State that external object class is a class. (modelica#2760)

* State that external object class is a class.
Closes modelica#2705

chapters/functions.tex

@@ -2524,6 +2524,11 @@ \subsection{External Objects}\label{external-objects}
   and after the function call).  Normally this is an external function, but it could be a non-external function as well (e.g.\ calling external functions one
   or more times).  The function input shall not have a default value using the constructor.
   \end{nonnormative}
+\item
+  An external object class shall be of the specialized class \lstinline!class!.
+  \begin{nonnormative}
+  This is the only use of \lstinline!class!.
+  \end{nonnormative}
 \item
   Classes derived from \lstinline!ExternalObject! can neither be used in an
   extends-clause nor in a short class definition.