docs: note before/afterUpdate breaking change (#14567)

...about slotted content behavior
Related to #14564

Author: dummdidumm

documentation/docs/07-misc/07-v5-migration-guide.md

@@ -823,6 +823,8 @@ The `foreign` namespace was only useful for Svelte Native, which we're planning
 
 `afterUpdate` callbacks in a parent component will now run after `afterUpdate` callbacks in any child components.
 
+`beforeUpdate/afterUpdate` no longer run when the component contains a `<slot>` and its content is updated.
+
 Both functions are disallowed in runes mode — use `$effect.pre(...)` and `$effect(...)` instead.
 
 ### `contenteditable` behavior change