docs: note default value breaking change to bind: in runes mode (#12412)

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

#11400 (comment)

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

Co-authored-by: Rich Harris <[email protected]>

---------

Co-authored-by: Rich Harris <[email protected]>

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 bindable property 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 ambiguous behavior — the parent and child must have the same value — and results in better performance (in Svelte 4, the default value was reflected back to the parent, resulting in wasteful additional 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.