bhyve.8: Correct description for -c flag, tag spdx

The examples only show the usage of -c <numcpus>, as did the flag
description, however the -c flag supports more complex cpu topology
specifiers. These were documented correctly in SYNOPSIS, add them to
the body of the DESCRIPTION as well. Someone could go further and do
and example with using them.

Approved by: re (cperciva)
MFC after: 3 days (there was a merge conflict with the date)
Event: OpenZFS Developer Summit '25
Reported by: Levi Worley <levi@gainframe.com>

(cherry picked from commit 205af037e302fbd50dabc485a89e2222cd063b9e)
(cherry picked from commit 772837c632ec9f3cb3ded33830a60a8b310c2e22)