docs: note default value breaking change to bind: in runes mode

#11400 (comment)

Author: dummdidumm

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -171,6 +171,8 @@ Exports from runes mode components cannot be bound to directly. For example, hav
 
 In Svelte 4 syntax, every property (declared via `export let`) is bindable, meaning you can `bind:` to it. In runes mode, properties are not bindable by default: you need to denote bindable props with the [`$bindable`](/docs/runes#$bindable) rune.
 
+If a property is bindable and has a default value (e.g. `let { foo = $bindable('bar') } = $props();`), you need to pass a non-`undefined` value to that property, if you're binding to it. This prevents undefined behavior and results in more performant code (in Svelte 4, the default value was propagated back up, which on the server and client could result in more render cycles).
+
 ### `accessors` option is ignored
 
 Setting the `accessors` option to `true` makes properties of a component directly accessible on the component instance. In runes mode, properties are never accessible on the component instance. You can use component exports instead if you need to expose them.