gif.4: Mention that gif(4) can tunnel Ethernet traffic

Ideally this manpage shall have examples for the EtherIP setup, but
there has been one in if_bridge(4) since the introduction of EtherIP [1],
so direct the setup to if_bridge(4) at this moment.

While here, add RFC 3378 to the reference section.

[1] 73ff045c575b Add RFC 3378 EtherIP support

Reviewed by: meta
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51781