Release notes for generic component propagation (#20354)

# Objective

Fixes #20143.

## Solution

These release notes were adopted from the PR description and docs added
in #17575 by @robtfm.

When writing these, I've focused on providing context, potential
applications, and breaking down the complex generics for newer Bevy
users.

Author: alice-i-cecile

crates/bevy_app/src/propagate.rs

@@ -30,6 +30,11 @@ use bevy_reflect::Reflect;
 /// to reach a given target.
 /// Individual entities can be skipped or terminate the propagation with the [`PropagateOver`]
 /// and [`PropagateStop`] components.
+///
+/// Propagation occurs during [`Update`] in the [`PropagateSet<C>`] system set.
+/// You should be sure to schedule your logic relative to this set: making changes
+/// that modify component values before this logic, and reading the propagated
+/// values after it.
 pub struct HierarchyPropagatePlugin<
     C: Component + Clone + PartialEq,
     F: QueryFilter = (),

release-content/release-notes/generic_component_propagation.md

@@ -0,0 +1,34 @@
+---
+title: Generic component propagation
+authors: ["@robtfm"]
+pull_requests: [17575]
+---
+
+When working with large hierarchies of game objects, coordinating the state of the entire tree can be frustrating.
+Bevy uses this pattern when working with transforms and visibility internally,
+but users have had to reinvent the wheel every time they wanted to use similar patterns.
+
+While this pain was most acute when working with [`RenderLayers`], this pattern is more broadly useful,
+and has been exposed to end users in the form of the [`HierarchyPropagatePlugin`].
+You might use this for synchronizing color and alpha values for "ghost" versions of previewed buildings,
+ensuring that all of the parts of a model are on the same render layer,
+or propagating font styles.
+
+This plugin has three generics:
+
+- `C: Component`: the type of component that should be propagated
+- `F: QueryFilter=()`: if set, only entities which match this filter will be affected by propagation
+- `R: Relationship = ChildOf`: the type of tree-like relationship to propagate down
+
+Each copy of this plugin will propagate components of type `C` down the hierarchy, along all entities which match the
+query filter of type `F`.
+With this plugin enabled for `C`, you can add a [`Propagate<C>`] component to add new components to all children,
+add a [`PropagateStop<C>`] component to stop propagation, or even use [`PropagateOver<C>`] to skip this entity during propagation.
+
+This is a very general tool: please let us know what you're using it for and we can continue to add examples to the docs!
+
+[`RenderLayers`]: https://dev-docs.bevy.org/bevy/camera/visibility/struct.RenderLayers.html
+[`HierarchyPropagatePlugin`]: https://dev-docs.bevy.org/bevy/app/struct.HierarchyPropagatePlugin.html
+[`Propagate<C>`]: https://dev-docs.bevy.org/bevy/app/struct.Propagate.html
+[`PropagateStop<C>`]: https://dev-docs.bevy.org/bevy/app/struct.PropagateStop.html
+[`PropagateOver<C>`]: https://dev-docs.bevy.org/bevy/app/struct.PropagateOver.html