Add note about Over/Out hierarchy issues

Author: NthTensor

crates/bevy_picking/src/events.rs

@@ -350,6 +350,11 @@ pub struct PickingEventWriters<'w> {
 /// receive [`Out`] and then entity B will receive [`Over`]. No entity will ever
 /// receive both an [`Over`] and and a [`Out`] event during the same frame.
 ///
+/// When we account for event bubbling, this is no longer ture. When focus shifts
+/// between children, parent entities may receive reduntant [`Out`] → [`Over`] pairs.
+/// In the context of UI, this is especially problematic. Additional hierarchy-aware
+/// events will be added in a future release.
+///
 /// Both [`Click`] and [`Up`] target the entity hovered in the *previous frame*,
 /// rather than the current frame. This is because touch pointers hover nothing
 /// on the frame they are released. The end effect is that these two events can