Add release note for infiinite children (#21107)

Fixes #21052.

The changes I thought needed to happen to `related!` were actually
already made, so we're good to go.

---------

Co-authored-by: Carter Anderson <[email protected]>

Author: alice-i-cecile

release-content/release-notes/infinite-children.md

@@ -0,0 +1,15 @@
+---
+title: Infinite Children
+authors: ["@CorvusPrudens"]
+pull_requests: [18865]
+---
+
+The `children!` macro is a convenient way to spawn children alongside their parents in Bevy code.
+When it was introduced in **Bevy 0.16** this was limited to 12 children, due to arbitrary limitations (Rust: please [support variadic generics!](https://blog.rust-lang.org/inside-rust/2025/09/11/program-management-update-2025-08/#variadic-generics)), and not implementing the requisite workarounds.
+When working with large UI hierarchies, this could be a real nuisance, forcing users to resort to ugly workarounds.
+
+We've rewritten the macro and lifted this unjust restriction. You are now only limited by Rust's recursion limit: around 1400 children at once.
+Rejoice!
+If you are manually spawning more than 1400 children in a single macro call, you should reconsider your strategy (such as using `SpawnIter` or `SpawnWith`).
+
+We've made the same change to the `related!` macro, allowing you to spawn huge numbers of related entities in a single call.