Update documentation now that CVSup service has been shutdown.

PR:		194916
Differential Revision:	https://reviews.freebsd.org/D1137
Approved by:	emaste, remko (mentor)


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@45969 c2e8774f-c49f-e111-b436-862b2bbc8956