Improve ComboBox doc example (#7116)

Improves the `ComboBox` example with some code that shows how to handle
changes in the `ComboBox`’s selection. The approach is based on the
advice given in https://github.com/emilk/egui/discussions/923 . I hope
this saves future me (and hopefully others) a web search for how to do
this.

* [x] I have followed the instructions in the PR template

crates/egui/src/containers/combo_box.rs

@@ -16,9 +16,10 @@ pub type IconPainter = Box<dyn FnOnce(&Ui, Rect, &WidgetVisuals, bool)>;
 ///
 /// ```
 /// # egui::__run_test_ui(|ui| {
-/// # #[derive(Debug, PartialEq)]
+/// # #[derive(Debug, PartialEq, Copy, Clone)]
 /// # enum Enum { First, Second, Third }
 /// # let mut selected = Enum::First;
+/// let before = selected;
 /// egui::ComboBox::from_label("Select one!")
 ///     .selected_text(format!("{:?}", selected))
 ///     .show_ui(ui, |ui| {
@@ -27,6 +28,10 @@ pub type IconPainter = Box<dyn FnOnce(&Ui, Rect, &WidgetVisuals, bool)>;
 ///         ui.selectable_value(&mut selected, Enum::Third, "Third");
 ///     }
 /// );
+///
+/// if selected != before {
+///     // Handle selection change
+/// }
 /// # });
 /// ```
 #[must_use = "You should call .show*"]