Revisit and document definition keys

I've done so. I've added a 'constraints' sub-array which can contain type-specific constraints and 'settings' for implementation specific settings - as used for computed properties. I've also moved 'entity type' and 'bundle' below constraints, as those are type specific. That way, all definition keys are defined fixed and may be extend depending on usage, i.e. entity properties add some specific keys (queryable, translatable). The same way additional keys could be added when data definitions are used to describe condition parameters, etc.