SDL example: more cleanup to match other examples (#233 #226)

examples/sdl_opengl_example/imgui_impl_sdl.h

@@ -1,3 +1,6 @@
+// ImGui SDL2 binding with OpenGL
+// https://github.com/ocornut/imgui
+
 struct SDL_Window;
 typedef union SDL_Event SDL_Event;
 
@@ -6,5 +9,6 @@ void        ImGui_ImplSdl_Shutdown();
 void        ImGui_ImplSdl_NewFrame(SDL_Window *window);
 bool        ImGui_ImplSdl_EventCallback(const SDL_Event& event);
 
+// Use if you want to reset your rendering device without losing ImGui state.
 void        ImGui_ImplSdl_InvalidateDeviceObjects();
 bool        ImGui_ImplSdl_CreateDeviceObjects();