Collection name -> array definition:
['acmeDatabaseName' => ['acmeCollectionName' => ['class' => '\Acme\Collection\SomeCollectionClass']]]
Collection name -> collection class name (deprecated: use definition array):
['acmeDatabaseName' => ['acmeCollectionName' => '\Acme\Collection\SomeCollectionClass']]
Collection's class namespace (deprecated: use definition array):
['acmeDatabaseName' => '\Acme\Collection']