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

+ update README.

backends/imgui_impl_win32.cpp

@@ -488,10 +488,10 @@ static ImGuiKey ImGui_ImplWin32_VirtualKeyToImGuiKey(WPARAM wParam)
 #endif
 
 // Win32 message handler (process Win32 mouse/keyboard inputs, etc.)
-// Call from your application's message handler.
+// Call from your application's message handler. Keep calling your message handler unless this function returns TRUE.
 // When implementing your own backend, you can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if Dear ImGui wants to use your inputs.
-// - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application.
-// - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application.
+// - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application, or clear/overwrite your copy of the mouse data.
+// - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application, or clear/overwrite your copy of the keyboard data.
 // Generally you may always pass all inputs to Dear ImGui, and hide them from your application based on those two flags.
 // PS: In this Win32 handler, we use the capture API (GetCapture/SetCapture/ReleaseCapture) to be able to read mouse coordinates when dragging mouse outside of our window bounds.
 // PS: We treat DBLCLK messages as regular mouse down messages, so this code will work on windows classes that have the CS_DBLCLKS flag set. Our own example app code doesn't set this flag.