fix: change detection example

examples/ecs/component_change_detection.rs

@@ -40,11 +40,25 @@ fn change_component(time: Res<Time>, mut query: Query<(Entity, &mut MyComponent)
 // Only entities matching the filters will be in the query
 fn change_detection(query: Query<(Entity, Ref<MyComponent>), Changed<MyComponent>>) {
     for (entity, component) in &query {
+        // By default you can only what component was changed on each entity. This is useful, but
+        // what if you have multiple systems modifying the same component?
+        #[cfg(not(feature = "track_change_detection"))]
         info!(
-            "{:?} changed: {:?} by: {}",
+            "{:?} changed {:?}",
             entity,
             component,
-            component.changed_by()
+        );
+
+        // If you enable the `track_change_detection` feature, you can unlock the
+        // `Ref::changed_by()` method. It returns the `Location`, the file and line number, that
+        // the component was changed in. It's not recommended for released games, but great for
+        // debugging!
+        #[cfg(feature = "track_change_detection")]
+        info!(
+            "{:?} changed {:?} in {}",
+            entity,
+            component,
+            component.changed_by(),
         );
     }
 }