bhyve: Document the hw.vmm.maxcpu tunable and the current limit on vCPUs

Reviewed by: corvink (original version)
Co-authored-by: John Baldwin <jhb@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D40074

(cherry picked from commit da202b0fe616e9314739f01493ae310e37a36d8d)