[Backport v25-branch] docs: clarify WordPress 404 page caching

docs/page-caching.md

@@ -53,9 +53,12 @@ Pages which can be cached have default time-to-live (TTL) values within the cach
 
 - Success responses (`200`): 300 seconds (5 minutes)
 - Client errors (`400`, except `404`): 10 seconds
-- Not found errors (`404`): 300 seconds (5 minutes)
+- Not found errors (`404`) generated by nginx before the request reaches WordPress: 300 seconds (5 minutes)
 - Server errors (`5XX`): 300 seconds (5 minutes)
 
+WordPress-generated 404 responses are not cached by default. WordPress sends `no-cache` headers for these responses, and Altis does
+not cache responses with a `Cache-Control: no-cache` header.
+
 Time-to-live values should be balanced between achieving high cache hit rates and ensuring fresh content is served.
 
 This can be adjusted by sending
@@ -68,6 +71,20 @@ For example, to specify a lifetime of one hour for a specific page:
 header( 'Cache-Control: max-age=' . HOUR_IN_SECONDS );
 ```
 
+For high-traffic 404 pages generated by WordPress, you can override WordPress' default `no-cache` headers and allow the CDN to cache
+the response by sending an explicit `Cache-Control` header:
+
+```php
+add_action( 'template_redirect', function () {
+	if ( ! is_404() ) {
+		return;
+	}
+
+	$lifetime = 5 * MINUTE_IN_SECONDS;
+	header( 'Cache-Control: s-maxage=' . $lifetime . ', max-age=' . $lifetime . ', must-revalidate' );
+} );
+```
+
 (Note that headers should be sent before any output occurs on the page.)
 
 ## Developer Considerations