swi(9): clean up description of clk_intr_event

[mhorne]

Summary

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.

[markj]

You might note in hardclock.9 that it's specifically the clk_intr_event handlers that get scheduled from hardclock(). Also, softclock is now decoupled from swi so the line "Schedule softclock interrupts ( swi(9)) processing" is somewhat misleading. softclock threads are scheduled from handleevents(), not hardclock().