In addition to a moderator's manual, I opened a moderator forum and requested that the moderators post a note after each message they moderate or remove... and I do the same, so it's a very good way for them to learn quickly, and for me to comment on what they did (say it was correct, incorrect, questionable, etc).
A moderator's guide that explains how to use all of the moderator's features in this program would be useful, but I have found that all you really need to explain is what applies to your particular forum.
You should have both a concept and how-to section on each function a moderator should do, with the concept being unique to your moderation policies/practices, and the how-to being a step-by-step technical reference guide for each task.
