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

(cherry picked from commit bf6027df0955cb79c9c56c47cc96709b2215c82a)
(cherry picked from commit 1b42526ce9f4e4587f4179a7e78655ce34b95110)