Sync svelte docs (#750)

sync svelte docs

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

Author: github-actions[bot]

apps/svelte.dev/content/docs/svelte/07-misc/04-custom-elements.md

@@ -49,6 +49,8 @@ console.log(el.name);
 el.name = 'everybody';
 ```
 
+Note that you need to list out all properties explicitly, i.e. doing `let props = $props()` without declaring `props` in the [component options](#Component-options) means that Svelte can't know which props to expose as properties on the DOM element.
+
 ## Component lifecycle
 
 Custom elements are created from Svelte components using a wrapper approach. This means the inner Svelte component has no knowledge that it is a custom element. The custom element wrapper takes care of handling its lifecycle appropriately.