[Config] Add a better doc for enum nodes

enum nodes have been added in #1826 but there is no proper documentation on how to use it, an example could be great:

``` php
<?php
$parent
    ->enumNode('node-name')
        ->values(array('a', 'b', 'c'))
    ->end()
```

ref: http://symfony.com/doc/master/components/config/definition.html

/cc @drak
