intr_event(9): update existing function descriptions

Document new arguments and behaviours for these functions as compared to
the old ithread_* versions.

Reviewed by: pauamma
Input from: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33478

(cherry picked from commit cb9425e21c917732fb16bd14947cba01f26687f3)