re.4: Mention vendor driver for if_re(4)

If this if_re(4) is causing problems then an updated driver from
the vendor can be found in the ports tree under net/realtek-re-kmod.

Reviewed by: debdrup, koobs, emaste
Differential Revision: https://reviews.freebsd.org/D33677

(cherry picked from commit a9a35c9e35370d09be5cbcd32f287b12f40812a5)