Add warning about SSR features inside layouts

src/pages/en/guides/server-side-rendering.md

@@ -47,6 +47,10 @@ You can find instructions at the individual adapter links above to complete the
 
 Astro will remain a static-site generator by default. But once you enable a server-side rendering adapter, **every route in your pages directory becomes a server-rendered route** and a few new features become available to you.
 
+:::warning
+The features below are not available inside layouts nor components. Because of streaming, things like headers and status codes can only be sent once, and by the time a layout or component is rendered, this had already happened.
+:::
+
 ### `Astro.request.headers`
 
 The headers for the request are available on `Astro.request.headers`. It is a [Headers](https://developer.mozilla.org/en-US/docs/Web/API/Headers) object, a Map-like object where you can retrieve headers such as the cookie.