caching.md, false examples: updated

The previous exmaple for adding a Mamcached backend was not working properly. Using this example results in Zend_Cache_Backend_Memcached using the default (fallback) settings (which are exactly the same as in the exmaple though).

+ Added an extra example for Memcached instances which are listening on a Unix socket.

docs/en/02_Developer_Guides/08_Performance/01_Caching.md

@@ -131,6 +131,28 @@ To use this backend, you need a memcached daemon and the memcache PECL extension
 	);
 	SS_Cache::pick_backend('primary_memcached', 'any', 10);
 	
+If your Memcached instance is using a local Unix socket instead of a network port:
+
+ 	:::php
+	// _config.php 
+	SS_Cache::add_backend(
+		'primary_memcached',
+		'Memcached',
+		array(
+		  	'servers' => array(
+				'host' => 'unix:///path/to/memcached.socket',
+				'port' => 0,
+				'persistent' => true,
+				'weight' => 1,
+				'timeout' => 5,
+				'retry_interval' => 15,
+				'status' => true,
+				'failure_callback' => null
+			)
+		)
+	);
+	SS_Cache::pick_backend('primary_memcached', 'any', 10);
+
 ### APC
 
 This backends stores cache records in shared memory through the [APC](http://pecl.php.net/package/APC)