Misc cleanup (#3935)

* Improve docstring
* Nicer welcome gif in README
* Misc cleanup

crates/eframe/src/lib.rs

@@ -293,7 +293,7 @@ pub fn run_native(
 ///                     .labelled_by(name_label.id);
 ///             });
 ///             ui.add(egui::Slider::new(&mut age, 0..=120).text("age"));
-///             if ui.button("Click each year").clicked() {
+///             if ui.button("Increment").clicked() {
 ///                 age += 1;
 ///             }
 ///             ui.label(format!("Hello '{name}', age {age}"));