Fix ifa_data description.

PR:		176583
Discussed with:	glebius
MFC after:	1 week
Sponsored by:	Nginx, Inc.
callout(9): add sbt flavors of callout_schedule

Differential Revision:	https://reviews.freebsd.org/D1161 (partial)
Reviewed by:	jhb, markj
MFC after:	1 week