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

+ update README.

examples/example_sdl_metal/main.mm

@@ -88,8 +88,8 @@ int main(int, char**)
         {
             // Poll and handle events (inputs, window resize, etc.)
             // 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.
             SDL_Event event;
             while (SDL_PollEvent(&event))