bhyve: Document vCPU range pinning

This change documents the recently introduced changes to -p
that allow users to specify CPU ranges instead of having to
specify each individual mapping.

While we're here, move the -p examples to the EXAMPLES section.

Reviewed by: bcr
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D57480

(cherry picked from commit 23c99b64918eddb6084ffe4347faf95f82661c47)