bridge.4: Improve VLAN documentation

Document how bridge implements VLANs, and provide an example of
configuring a bridge with VLAN filtering.

MFC after: 3 days
Reviewed by: ziaee, pauamma_gundo.com (previous version)
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D51185