Update cipher-related man page text

Update cipher-related man page text

As reported in trac #732, the man page text for --cipher is no longer
accurate.  Update the text to represent current knowledge, about NCP and
SWEET32.

This does not hint at changing the default cipher, because we did not make
a decision on that yet.  If we do change the default cipher, we'll have to
update the text to reflect that.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1473605431-20842-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12439.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>