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

+ update README.

examples/example_sdl_opengl3/main.cpp

@@ -101,8 +101,8 @@ 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))