FAQ #15: Can I edit/delete/screen a comment I posted or that was posted in my journal?
Although the FAQ Can I edit/delete/screen a comment I posted or that was posted in my journal? mentions (at the very bottom) that comments cannot be retrieved once deleted, this mention is rather parenthetical and not very ... noticeable.
I suggest to add the following sentence/paragraph:
Once deleted, a comment can not be undeleted because the comment's text is permanently erased from the LiveJournal servers.to the end of the first bit about deleting comments, i.e. just above "You can screen a comment....."