Document that window must be created in ShowScrollbars()

Calling it before creating the window actually works under MSW, but not
under the other platforms.

interface/wx/scrolwin.h

@@ -332,6 +332,8 @@ public:
             - wxSHOW_SB_DEFAULT: To restore the default behaviour described
                 above.
 
+        Note that the window must be created before calling this method.
+
         @param horz
             The desired visibility for the horizontal scrollbar.
         @param vert