Codechange: Make documentation comments for some types recognised by doxygen.

src/viewport_gui.cpp

@@ -50,6 +50,7 @@ static constexpr std::initializer_list<NWidgetPart> _nested_extra_viewport_widge
 	EndContainer(),
 };
 
+/** Extra viewport window. */
 class ExtraViewportWindow : public Window {
 public:
 	ExtraViewportWindow(WindowDesc &desc, int window_number, TileIndex tile) : Window(desc)