FAQ 62 covers how to enable or disable commenting. It also discusses how to find and read comments, but it doesn't say how to *post* comments. In the bottom section, it says that it's possible to reply directly to another comment, but doesn't say exactly how to do it.
FAQ 23 covers how to control the comments that are allowed in the journal. A user who is having problems commenting would need to backwards-engineer the sentences to infer why she can't get her comment posted.
FAQ 122 is "why can't I reply to comments via email?" but there isn't a FAQ there for "why won't my comment post?"
So... I see the need for two related FAQs or FAQ subsections.
The first would be how to post comments. It would talk about clicking on the "Leave a comment" link in Recent Entries (noting that said link could be modified/customized by the journal owner, and isn't there some icon that can also be used in some styles-- if so, show what it looks like). It would also discuss the "Post a new comment" link in the Entry View.
A related part of this would be to say that following the above will post a new top-level comment in response to the entry. If one wanted to reply to a comment posted to the entry, go to the Entry View, find the comment you want to respond to, click on the (Reply to this) link. Note that this causes comments to thread; if necessary, leave a pointer to the section in FAQ 62 that talks about comment threading. I see many new users posting new comments directly to the entry when they are trying to reply to a comment someone else has already left.
The second FAQ would troubleshoot what if you can't post a comment. Reasons being: person not allowing any comments, person allowing only friends-only comments, person allowing only registered users or non-anonymous comments, checking if you're logged in, problems with OpenID, and pointers to the relevant related FAQs or pages such as login.bml.
[and OMG I'm an idiot-- I just noticed that FAQ #207 *does* include
Comment on someone else's journal entry?
Click on the "post comment" link for that particular entry. (Note: Some users may have changed their comment link text.)
... but is that really intuitive enough? I'm not sure. Even so, I still think a troubleshooting FAQ dealing with comments would be a nice thing.]