Add memcached cache config tips and examples, including Unix sockets. Link to Redis config section.

[A previous pull request](https://github.com/laravel/laravel/pull/3008) explained the background.

[OT suggested putting this info in the docs](https://github.com/laravel/laravel/pull/3008#issuecomment-55362861), instead of the config.php comments, so here it is.