docs: add examples of when to use portal

Author: hperrin

packages/site/src/routes/demo/menu/+page.svelte

@@ -32,7 +32,7 @@
     Portal
     <svelte:fragment slot="subtitle">
       Using a svelte-portal to show a menu in a place where it normally couldn't
-      go.
+      go (due to DOM restrictions, hidden overflow, etc.).
     </svelte:fragment>
   </Demo>