Add better documentation for epi::NativeOptions::drag_and_drop_support

Closes https://github.com/emilk/egui/issues/747

egui_demo_app/src/main.rs

@@ -9,6 +9,7 @@ fn main() {
     let options = eframe::NativeOptions {
         // Let's show off that we support transparent windows
         transparent: true,
+        drag_and_drop_support: true,
         ..Default::default()
     };
     eframe::run_native(Box::new(app), options);