Specify prototypes in Include-files, in particular const-ness.

Closes modelica#2278

chapters/functions.tex

@@ -1738,6 +1738,12 @@ \section{External Function Interface}\doublelabel{external-function-interface}
 is treated the same way as a top-level input or output in the external
 call.
 
+If the function has \lstinline!annotation(Include="includeDirective")!, \autoref{annotations-for-external-libraries-and-include-files} 
+it is assumed that it contains an actual prototype and no prototype shall be automatically generated.
+In that case the input argument pointers shall be const pointers (indicating that they do not modify the inputs), 
+and non-const pointers are deprecated.
+The optional external-function-call is still used for determining the name of the function, and order of arguments, as described below.
+
 \subsection{Argument type Mapping}\doublelabel{argument-type-mapping}
 
 The arguments of the external function are declared in the same order as