The manual needs to discuss how to get started, how to use the clients, how to find other people, how to manage friends lists, how to start, join, and use communities and topics, how to use more advanced features such as styles and themes and polls, how to learn more, use support, make suggestions, and different ways to volunteer in the community (coding, support, writing).
That's just off the top of my head. I'm probably missing stuff.
To make this happen, we'd need a sufficiently nitpicky volunteer to edit the thing, a sufficiently tenacious volunteer to maintain it (track LJ changes and get sections added or updated as needed), and a bunch of volunteers who are sufficiently dedicated to learn everything about feature X and write a clear and concise manual section about it.
Now, this should be do-able. We have lots of talented volunteers, we certainly have no shortage of people who like to write :-), and thanks to LiveJournal's community feature, we have a great way to organize effort. (And it'd make nice resume fodder for anyone interested in tech writing...)
Unfortunately I can't volunteer to coordinate the effort, because I'm already overcommitted between LJ code work and real-job work; so this will only happen if someone volunteers to step up and run the project.
So. Who's in?