Promoted the stale-while-revalidate and stale-if-error directives

lib/classes/Config.php

@@ -89,7 +89,7 @@ public static function getDefaultConfig($skipQualityAuto = false) {
 
             // serve options
             'cache-control' => 'no-header',     /* can be "no-header", "set" or "custom" */
-            'cache-control-custom' => 'private, max-age:3600',
+            'cache-control-custom' => 'public, max-age:86400, stale-while-revalidate=604800, stale-if-error=604800',
             'cache-control-max-age' => 'one-week',
             'cache-control-public' => false,
             'fail' => 'original',

lib/options/options/serve-options/cache-control.inc

@@ -9,8 +9,11 @@ $cacheControlPublic = $config['cache-control-public'];
 
 <tr>
     <th scope="row">Cache-Control header <?php echo helpIcon(
-        'Controls the cache-control header on successful conversion and direct redirection to converted ' .
-        'image in .htaccess. In case of convert failure, headers will be sent to prevent caching. ');
+        '<p>Controls the cache-control header on successful conversion and direct redirection to converted ' .
+        'image in .htaccess. In case of convert failure, headers will be sent to prevent caching.</p>' .
+        '<p>PS: In order to set <i>stale-while-revalidate</i> and <i>stale-if-error directives<i>, you must ' .
+        'currently choose "Custom". <a target="_blank" href="https://www.fastly.com/blog/stale-while-revalidate-stale-if-error-available-today">It is a good idea to set these</a>.' .
+        '</p>');
     ?>
     </th>
     <td>