Sometimes we (the FAQ editors) need to discuss things without input from lj_userdoc at large, and a single community is much easier to keep up with than scattered e-mails.
An analogy! lj_docadmin:lj_userdoc::lj_supportadmin:lj_support
Don't worry, the existence of this community won't really affect the way lj_userdoc runs: all FAQ proposals and changes will still go through this community.
If you've got questions about this in particular, feel free to comment.