DQL custom functions on doctrine reference page

Custom DQL functions can be configured in two ways, according to what I was told [ here ] (https://github.com/symfony/symfony-docs/pull/3939)

It should be clearified in doctrine reference page because It may confuse some people that just read the cookbook  section about [custom DQL functions] (http://symfony.com/doc/current/cookbook/doctrine/custom_dql_functions.html)
I think - to avoid duplication (but to keep things clear) we should only show the top-level items (e.g. `query_cache_driver`, `metadata_cache_driver`, etc) and put a `#...` under each if it has child options. So:

```yaml
query_cache_driver:
    # ...
metadata_cache_driver:
    # ...
result_cache_driver:
    # ...
connection: ~
class_metadata_factory_name: Doctrine\ORM\Mapping\ClassMetadataFactory
```

and then continued for everything else. What do you think about that?