swi(9): clean up description of clk_intr_event

From what I can tell, setdelayed() was removed so long ago that its
mention is more likely to be confusing than helpful. We now have a
manpage for hardclock(9), so reference that.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36937

(cherry picked from commit 96b76a6a1edd6d0e0d4fa07c37d11d5805e8b8b3)