intr_event(9): update top-level description

The ithread has been subsumed by the 'interrupt event' object, so
update the description to reflect this by describing an interrupt event
and its contents. We've also moved on from having a single handler
function to the split filter-and-handler model. Explain the purpose and
constraints of these two types of handlers.

Reviewed by: jhb, pauamma
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33477

(cherry picked from commit dfc91493abc6046fc6cb237e48fd952deee358e0)