Add a link to cache expiration `expires` docs

h5bp/web_performance/cache-control.conf

@@ -31,6 +31,7 @@
 #     control configuration is used or not.
 #     Cache expiration configuration `expires` is described in the file
 #     h5bp/web_performance/cache_expiration.conf.
+#     https://nginx.org/en/docs/http/ngx_http_headers_module.html#expires
 #
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
 # https://www.rfc-editor.org/rfc/rfc9111.html