Document that hidden class Fl_X is accessible only if FL_LIBRARY is #define'd.

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@8276 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

documentation/src/osissues.dox

@@ -291,7 +291,7 @@ Fl_X *Fl_X::set_xid(Fl_Window*, Window xid)
 \endverbatim
 
 \par
-Allocate a hidden structure called an Fl_X, put the
+Allocate a hidden class called an Fl_X, put the
 XID into it, and set a pointer to it from the Fl_Window.
 This causes Fl_Window::shown() to return true.
 
@@ -368,6 +368,9 @@ MyWindow::~MyWindow() {
 }
 \endcode
 
+\note Access to the Fl_X hidden class requires to \#define FL_LIBRARY
+before compilation.
+
 \subsection osissues_x_icon Setting the Icon of a Window
 
 FLTK currently supports setting a window's icon \b before it