Deprecate the "builtin" external language

chapters/functions.tex

@@ -1712,14 +1712,14 @@ \section{External Function Interface}\label{external-function-interface}
 This is just as for any other function.  The components in the protected part allow local variables for temporary storage to be declared.
 \end{nonnormative}
 
-The \lstinline!language-specification! must currently be one of \lstinline!"builtin"!, \lstinline!"C"!, \lstinline!"C..."! (for one of the specific C standards like C89, C99, and C11 -- specifying
+The \lstinline!language-specification! must currently be one of \lstinline!"builtin"! (deprecated), \lstinline!"C"!, \lstinline!"C..."! (for one of the specific C standards like C89, C99, and C11 -- specifying
 that it relies on the C standard library of that version) or \lstinline!"FORTRAN 77"!.  Unless the external language is specified, it is assumed to be \lstinline!"C"!.
 
 \begin{nonnormative}
 The intended use of e.g.\ C99 is to detect if the user tries to link with a C99-function using a C89 compiler.
 \end{nonnormative}
 
-The \lstinline!"builtin"! specification is only used for functions that are defined
+The deprecated \lstinline!"builtin"! specification is only used for functions that are defined
 to be built-in in Modelica. The external-function call mechanism for
 \lstinline!"builtin"! functions is implementation-defined.
 

chapters/operatorsandexpressions.tex

@@ -621,6 +621,8 @@ \subsection{Built-in Mathematical Functions and External Built-in Functions}\lab
 \end{tabular}
 \end{center}
 
+These functions are the only ones that can also be called using the deprecated \lstinline!"builtin"! external language, see \cref{external-function-interface}.
+
 \begin{functiondefinition}[atan2]
 \begin{synopsis}\begin{lstlisting}
 atan2($y$, $x$)