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?
No. This is a nasty piece of imported code. We tend to leave problems
like that untouched, because otherwise it just creates a conflict next
time we update it.
There are very few places in the tree left with this.