boot(9): update to match reality

This function was renamed to kern_reboot() in 2010, but the man page has
failed to keep in sync. Bring it up to date on the rename, add the
shutdown hooks to the synopsis, and document the (obvious) fact that
kern_reboot() does not return.

Fix an outdated reference to the old name in kern_reboot(), and leave a
reference to the man page so future readers might find it before any
large changes.

Reviewed by: imp, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32085