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] 73ff045c57 Add RFC 3378 EtherIP support

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

share/man/man4/gif.4

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd July 14, 2025
+.Dd August 27, 2025
 .Dt GIF 4
 .Os
 .Sh NAME
@@ -55,6 +55,16 @@ does not perform GRE encapsulation; use
 .Xr gre 4
 for GRE encapsulation.
 .Pp
+The
+.Nm
+interface can also tunnel Ethernet traffic over IPv4 or IPv6
+when combined with a
+.Xr if_bridge 4
+interface using EtherIP protocol.
+See
+.Xr if_bridge 4
+for detailed setup.
+.Pp
 Each
 .Nm
 interface is created at runtime using interface cloning.
@@ -316,6 +326,13 @@ to the desired level of nesting.
 .%D December 1999
 .%O draft-ietf-ipsec-ecn-02.txt
 .Re
+.Rs
+.%A R. Housley
+.%A S. Hollenbeck
+.%T EtherIP: Tunneling Ethernet Frames in IP Datagrams
+.%R RFC 3378
+.%D September 2002
+.Re
 .\"
 .Sh HISTORY
 The