Better documentation for Event::Zoom (#4778)

* Closes #4777

Also add this to the 0.28 migration guide.

CHANGELOG.md

@@ -22,6 +22,7 @@ Changes since the last release can be found at <https://github.com/emilk/egui/co
 * `Ui::new` and `ui.child_ui` now takes a new parameter for the `UiStack` ([#4588](https://github.com/emilk/egui/pull/4588))
 * The `extra_asserts` and `extra_debug_asserts` feature flags have been removed ([#4478](https://github.com/emilk/egui/pull/4478))
 * Remove `Event::Scroll` and handle it in egui. Use `Event::MouseWheel` instead ([#4524](https://github.com/emilk/egui/pull/4524))
+* `Event::Zoom` is no longer emitted on ctrl+scroll. Use `InputState::smooth_scroll_delta` instead ([#4524](https://github.com/emilk/egui/pull/4524))
 * `ui.set_enabled` and `set_visbile` have  been deprecated ([#4614](https://github.com/emilk/egui/pull/4614))
 * `DragValue::clamp_range` renamed to `range` (([#4728](https://github.com/emilk/egui/pull/4728))
 

crates/egui/src/data/input.rs

@@ -426,10 +426,16 @@ pub enum Event {
     /// On touch-up first send `PointerButton{pressed: false, …}` followed by `PointerLeft`.
     PointerGone,
 
-    /// Zoom scale factor this frame (e.g. from ctrl-scroll or pinch gesture).
+    /// Zoom scale factor this frame (e.g. from a pinch gesture).
+    ///
     /// * `zoom = 1`: no change.
     /// * `zoom < 1`: pinch together
     /// * `zoom > 1`: pinch spread
+    ///
+    /// Note that egui also implement zooming by holding `Ctrl` and scrolling the mouse wheel,
+    /// so integration need NOT emit this `Zoom` event in those cases, just [`Self::MouseWheel`].
+    ///
+    /// As a user, check [`crate::InputState::smooth_scroll_delta`] to see if the user did any zooming this frame.
     Zoom(f32),
 
     /// IME Event

crates/egui/src/input_state.rs

@@ -27,6 +27,8 @@ const MAX_DOUBLE_CLICK_DELAY: f64 = 0.3; // TODO(emilk): move to settings
 
 /// Input state that egui updates each frame.
 ///
+/// You can access this with [`crate::Context::input`].
+///
 /// You can check if `egui` is using the inputs using
 /// [`crate::Context::wants_pointer_input`] and [`crate::Context::wants_keyboard_input`].
 #[derive(Clone, Debug)]