you want to look at the DocumentManager of phpcr-odm which is in this github project: https://github.com/doctrine/phpcr-odm/
the interface of fixture is using the base commons ObjectManager which is extended by orm / phpcr-odm and the others. this is why you don't see type hints if you use an IDE. but what you get there should be instanceof Doctrine\ODM\PHPCR\DocumentManager
if you want to use this with doctrine ORM, you would have to provide mappings for the Documents and probably some other work. we are currently working on that feature so it will come.