The document states this is temporary. The feature isn't even in the main systemd code yet, but is on a separate branch. They're avoiding specifying a format until they're sure that it's nailed down, does what they need, doesn't have some unforseen problem that could only be found through widespread testing, etc.
Once it's a stable feature, expect the format to be documented. Until then, the documentation is available in the .c files for anyone who has some need to avoid the provided library interface.