pfsync: Document the transport over IPv6 feature

On D40102 we implemented support for transport over IPv6 but the
documentation was not updated to reflect the new feature.

Clarify what is available and how it can be used.

MFC after: 1 week
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42505

(cherry picked from commit 81d4c786209bfa3752c25b2564eb363027f5d914)