Add detailed smarty cache documentation to config

Instead of just making it availble document the smarty cache feature.

It might work for users, but it's advised to rely on the memcache
instead.

Fixes #309

public/include/config/global.inc.dist.php

@@ -341,8 +341,30 @@ $config['cookie']['path'] = '/';
 $config['cookie']['name'] = 'POOLERCOOKIE';
 $config['cookie']['domain'] = '';
 
-// Disable or enable smarty cache
-// This is usually not required, default: 0
+
+/**
+ * Enable or disable the Smarty cache
+ *
+ * Explanation:
+ *   Smarty implements a file based cache for all HTML output generated
+ *   from dynamic scripts. It can be enabled to cache the HTML data on disk,
+ *   future request are served from those cache files.
+ *
+ *   This may or may not work as expected, in general Memcache is used to cache
+ *   all data so rendering the page should not take too long anyway.
+ *
+ *   You can test this out and enable (1) this setting but it's not guaranteed to
+ *   work with mmcfe-ng.
+ *
+ *   Ensure that the folder `templates/cache` is writable by the webserver!
+ *
+ *  Options:
+ *    0  =  disabled
+ *    1  =  enabled
+ *
+ *  Default:
+ *    0  =  disabled
+ **/
 $config['cache'] = 0;
 
 ?>