bhyve: Update usage and synopsis for the -k flag

Let's make it clear to users that -k is for configuration files.
Also, point to bhyve_config(5) in the paragraph describing the flag.

Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32467

(cherry picked from commit f656df586ad1aa86a2b9f076a224f746d5308613)