Cross reference tap(4) and tun(4) and include a short explanation as
to how they differ.  This will assist users in selecting which interface
is more appropriate for their purposes.

Approved by:	grog (co-mentor)
MFC after:	2 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275298 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f