Mention that wxEVT_MAXIMIZE is only generated by a few ports.

Currently only wxMSW, wxOSX/Cocoa and wxOS2 generate this event. Notably wxGTK
does not.

See #13022.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67157 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -1787,6 +1787,10 @@ public:
     not sent when the window is restored to its original size after it had been
     maximized, only a normal wxSizeEvent is generated in this case.
 
+    Currently this event is only generated in wxMSW, wxOSX/Cocoa and wxOS2
+    ports so portable programs should only rely on receiving @c wxEVT_SIZE and
+    not necessarily this event when the window is maximized.
+
     @beginEventTable{wxMaximizeEvent}
     @event{EVT_MAXIMIZE(func)}
         Process a @c wxEVT_MAXIMIZE event.