clarify using embedded XRC

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20738 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/txrc.tex

@@ -148,6 +148,8 @@ Use the {\tt -c} switch to
 contain a function called {\it InitXmlResource} (unless you override this with
 a command line switch). Use it to load the resource:
 \begin{verbatim}
+  extern void InitXMLResource(); // defined in generated file
+  ...
   wxXmlResource::Get()->InitAllHandlers();
   InitXmlResource();
   ...