Backends, Examples: Add comments about using WndProc handler and WantCaptureXXX flags. (#5061)

+ update README.

backends/imgui_impl_win32.h

@@ -22,6 +22,8 @@ IMGUI_IMPL_API void     ImGui_ImplWin32_NewFrame();
 // Win32 message handler your application need to call.
 // - Intentionally commented out in a '#if 0' block to avoid dragging dependencies on <windows.h> from this helper.
 // - You should COPY the line below into your .cpp code to forward declare the function and then you can call it.
+// - Call from your application's message handler. Keep calling your message handler unless this function returns TRUE.
+
 #if 0
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
 #endif