Well, the docs say that implicitly. Only Linux is supported as a server operating system (ownCloud Documentation Overview). As you can’t install Linux on a NTFS partition you can’t use ownCloud with it either. In the end it does not make sense to list all “supported/unsupported” file systems in the docs. You can only use ownCloud Server on Linux so you can only use the file systems that Linux runs on as a primary storage (apart from storages you connect via S3 but that’s another story).