I see that there exists a reference to an earlier mkisofs man page within the mkhybrid(1) man page, whereas no such man page seems to exist. There is a mkisofs.c which seems to be incorporated into mkhybrid nowadays, but I don't think there is any intention to support a man page for it.
Would submission of a diff to just eliminate the "(1)" reference, inside the man page for mkhybrid, be appropriate? Austin