git-cvsimport.txt: cvsps-2 is deprecated

git-cvsimport.txt: cvsps-2 is deprecated

git-cvsimport relies on version 2 of cvsps and does not work with the
new version 3.  Since cvsps 3.x does not currently work as well as
version 2 for incremental import, document this fact.

Specifically, there is no way to make new git-cvsimport that supports
cvsps 3.x and have a seamless transition for existing users since cvsps
3.x needs a time from which to continue importing and git-cvsimport does
not save the time of the last import or import into a specific namespace
so there is no safe way to calculate the time of the last import.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>