Yes, that’s what it refers to. userlandfs provides “real world” testing of the filesystem (you can easily open multiple files at the same time, for example, which is not so easy to do from the fs_shell).
It is documented here (very short documentation, and maybe not up to date): https://www.haiku-os.org/documents/dev/running_userlandfs/
I think it’s not very useful to start with this yet, if your filesystem is not working well in fs_shell. fs_shell is a more controlled environment and that’s better for the first steps.
For me personally, the documentation is more important than the code because it allows other people to understand things. When there is no documentation, everyone spends time to understand how things work, again and again. It is a waste of time for new people asking a lot of questions because the answers are not written anywhere, and for existing contributors because they have to explain the same thing over and over again.
As you can see in the files I linked, the docs are not like writing a full book about what you’re doing. It’s just short notes on some important things. And even small improvements there can be of great help for everyone to understand how things work.