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.

MFC after: 3 days
Event: OpenZFS Developer Summit '25
Reported by: Levi Worley <levi@gainframe.com>