Document that size of shaped windows can't be changed

Shaped windows have the size defined by the shape and it's not really
clear what calling SetSize() on them should do -- so just document that
it's not supposed to work.

Closes #9794.

interface/wx/nonownedwnd.h

@@ -37,6 +37,9 @@ public:
 
         This method is available in this class only since wxWidgets 2.9.3,
         previous versions only provided it in wxTopLevelWindow.
+
+        Note that windows with non default shape have a fixed size and can't be
+        resized using SetSize().
     */
     bool SetShape(const wxRegion& region);
 
@@ -55,6 +58,9 @@ public:
             SetShape(path);
         @endcode
 
+        Note that windows with non default shape have a fixed size and can't be
+        resized using SetSize().
+
         As the overload above, this method is not guaranteed to work on all
         platforms but currently does work in wxMSW, wxOSX/Cocoa and wxGTK (with
         the appropriate but almost always present X11 extensions) ports.