docs: Update Breaking changes section (#9444)

* update docs

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

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

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

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

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>
Co-authored-by: Rich Harris <hello@rich-harris.dev>

Author: 4 people

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

@@ -75,3 +75,13 @@ Previously Svelte would always insert the CSS hash last. This is no longer guara
 ### `contenteditable` behavior change
 
 If you have a `contenteditable` node with a corresponding binding _and_ a reactive value inside it (example: `<div contenteditable=true bind:textContent>count is {count}</div>`), then the value inside the contenteditable will not be updated by updates to `count` because the binding takes full control over the content immediately and it should only be updated through it.
+
+### `oneventname` attributes no longer accept string values
+
+In Svelte 4, it was possible to specify event attributes on HTML elements as a string:
+
+```svelte
+<button onclick="alert('hello')">...</button>
+```
+
+This is not recommended, and is no longer possible in Svelte 5, where properties like `onclick` replace `on:click` as the mechanism for adding [event handlers](/docs/event-handlers).