man9: Add man page for kern_yield() and friends

The page is fairly simple and will be referenced by mi_switch(9).
Provide some usage notes so that the broader implications of how/when to
use these functions are understood.

Reviewed by: kib, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38182