[Reference] add validation config reference section

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
What's going on here is that the extension class searches for a service with the id `validator.mapping.cache.<value>` where value is what the user configured with this option. The service class has to implement the `CacheInterface` from the Validator component.

My suggestion to reword this would be:

```rst
This value is used to determine the service that is used to persist class
metadata in a cache. The actual service name is built by prefix the configured
value with `validator.mapping.cache.`. The service has to implement the
:class:`Symfony\\Component\\Validator\\Mapping\\Cache\\CacheInterface`.
```

What do you think?