diff --git a/documentation/src/osissues.dox b/documentation/src/osissues.dox
index 95184dd29..e24935b04 100644
--- a/documentation/src/osissues.dox
+++ b/documentation/src/osissues.dox
@@ -82,6 +82,10 @@ Window fl_xid(const Fl_Window *)
 \par
 Returns the XID for a window, or zero if not \c shown().
 
+\par
+You must \#define FL_INTERNALS above all FLTK related \#include's
+to access this function.
+
 Fl_Window *fl_find(ulong xid)
 
 \par
@@ -516,6 +520,10 @@ HWND fl_xid(const Fl_Window *)
 Returns the window handle for a Fl_Window, or zero
 if not \c shown().
 
+\par
+You must \#define FL_INTERNALS above all FLTK related \#include's
+to access this function.
+
 Fl_Window *fl_find(HWND xid)
 
 \par
@@ -697,6 +705,10 @@ Returns the window reference for an Fl_Window, or
 \c NULL if the window has not been shown. This reference is a pointer
 to an instance of the subclass FLWindow of Cocoa's NSWindow class.
 
+\par
+You must \#define FL_INTERNALS above all FLTK related \#include's
+to access this function.
+
 Fl_Window *fl_find(Window xid)
 
 \par