docs: elaborate on setHeaders in SSR (#12888)

* docs: elaborate on setHeaders in SSR

* Update documentation/docs/20-core-concepts/20-load.md

* docs: update expression

* Update documentation/docs/20-core-concepts/20-load.md

---------

Co-authored-by: Tee Ming <[email protected]>

Author: hyunbinseo

documentation/docs/20-core-concepts/20-load.md

@@ -322,8 +322,8 @@ export async function load({ fetch, setHeaders }) {
 	const url = `https://cms.example.com/products.json`;
 	const response = await fetch(url);
 
-	// cache the page for the same length of time
-	// as the underlying data
+	// Headers are only set during SSR, caching the page's HTML
+	// for the same length of time as the underlying data.
 	setHeaders({
 		age: response.headers.get('age'),
 		'cache-control': response.headers.get('cache-control')