One possibility would be to throw in a paragraph before the last one, something like:
If you are manually entering the time of an entry, please note that the Update Journal page uses a 24-hour clock. This means that if you enter "8:00", LiveJournal will interpret this as 8am; in order to have 8pm instead, you will need to enter "20:00".
...and actually, there are a few minorer things that are annoying me about the FAQ, but that might just be me feeling cranky, so I won't bring 'em up. :) And won't, like, rewrite the entire thing, which I think I mainly feel the urge to do only because it's something /other/ than grading papers.
Of course, if the FAQ's fine the way it is, feel free to just tell me to go away :)