Provide basic descriptions for VMX exit reason (from "Intel 64 and IA-32
Architectures Software Developer’s Manual Volume 3"). Add the document
to SEE ALSO in bhyve.8 (and pet manlint here a bit).

Reviewed by: jhb, rgrimes, 0mp
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D17531