referenced documents need to be referenceable

currently documents referenced need to have referenceable=true in their document annotation, otherwise the @ReferenceOne silently fails. (see also https://github.com/symfony-cmf/cmf-sandbox/pull/29 )

either phpcr-odm should complain. or it could just add the mix:referenceable to the target document even though it was not declared referenceable.
we could additionally make referenceable=true the default and offer explicit referenceable=false for optimization.