Adding doc block info for Cache::config.

cake/libs/cache.php

@@ -89,6 +89,21 @@ function &getInstance() {
  *
  * `Cache::config('default');`
  *
+ * The following keys are used in core cache engines:
+ *
+ * - `duration` Specify how long items in this cache configuration last.
+ * - `prefix` Prefix appended to all entries. Good for when you need to share a keyspace
+ *    with either another cache config or annother application.
+ * - `probability` Probability of hitting a cache gc cleanup.  Setting to 0 will disable 
+ *    cache::gc from ever being called automatically.
+ * - `servers' Used by memcache. Give the address of the memcached servers to use.
+ * - `compress` Used by memcache.  Enables memcache's compressed format.
+ * - `serialize` Used by FileCache.  Should cache objects be serialized first.
+ * - `path` Used by FileCache.  Path to where cachefiles should be saved.
+ * - `lock` Used by FileCache.  Should files be locked before writing to them?
+ * - `user` Used by Xcache.  Username for XCache
+ * - `password` Used by Xcache.  Password for XCache
+ *
  * @see app/config/core.php for configuration settings
  * @param string $name Name of the configuration
  * @param array $settings Optional associative array of settings passed to the engine