docs: mention island features SFC only

docs/2.guide/2.directory-structure/1.components.md

@@ -295,6 +295,10 @@ Now you can register server-only components with the `.server` suffix and use th
 
 Server-only components use [`<NuxtIsland>`](/docs/api/components/nuxt-island) under the hood, meaning that `lazy` prop and `#fallback` slot are both passed down to it.
 
+::alert{type=warning}
+Most features for server only components and island components such as slots and client components are only available with SFC.
+::
+
 #### Client components within server components
 
 ::alert{type=info}