Revision 362996

MFC r362779: gre(4): Add a STANDARDS section

Expand the mentioned RFC in the SEE ALSO section
and reference RFC1701 and RFC1702.

PR:	240250
Reviewed by:	bcr (mentor)
Approved by:	bcr (mentor)
Obtained from:	OpenBSD
Differential Revision:	https://reviews.freebsd.org/D25504