Add reference to where "builtin" external language is used

chapters/functions.tex

@@ -1720,9 +1720,8 @@ \section{External Function Interface}\label{external-function-interface}
 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 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.
+The deprecated \lstinline!"builtin"! specification is only used for the elementary mathematical functions described in \cref{built-in-mathematical-functions-and-external-built-in-functions}.
+The external-function call mechanism for \lstinline!"builtin"! functions is implementation-defined.
 
 \begin{nonnormative}
 Typically, for functions from the standard C library, the prototype of the function is provided but no library annotation. Currently, there are