swi(9): remove BUGS section

Most of these globals have been removed, save for clk_intr_event. This
one is appropriate to keep in sys/interrupt.h, despite the fact that it
has only one consumer.

Bump .Dd for this and previous changes.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36939