docs: handle immutable headers error (#12543)

* docs: handle immutable headers error

* Update documentation/docs/30-advanced/20-hooks.md

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

---------

Co-authored-by: Tee Ming <chewteeming01@gmail.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

documentation/docs/30-advanced/20-hooks.md

@@ -67,6 +67,13 @@ export async function handle({ event, resolve }) {
 	event.locals.user = await getUserInformation(event.cookies.get('sessionid'));
 
 	const response = await resolve(event);
+
+	// Note that modifying response headers isn't always safe.
+	// Response objects can have immutable headers
+	// (e.g. Response.redirect() returned from an endpoint).
+	// Modifying immutable headers throws a TypeError.
+	// In that case, clone the response or avoid creating a
+	// response object with immutable headers.
 	response.headers.set('x-custom-header', 'potato');
 
 	return response;