My only concerns about having a man page is that eventually the configuration file (in this case rc.conf) might gradually become less well commented, or it's comments become outdated. And that man pages tend to be long on highly technical explanations that I for one have a hard time understanding and are often short on examples. So rather than having the rc.conf refer to a man page for instruction on how to use it. I'd much prefer that the primary method of "guidance" remain in the rc.local and perhaps include in any man page a url from which one can download a current rc.local.example file. +1 No matter man pages become quite useful (and big) rc's files should still be commented because this is very handy when editing them.
