Update comments referencing create_thread()

The equivalent function is now named thread_create(). Mention
kthread_add() where it is also relevant.

Reviewed by: kib, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38180

(cherry picked from commit a7a452fedc42d9e260224b7e2cd833df7287319f)