Improve sections about older OpenVPN clients in cipher-negotiation.rst

Improve sections about older OpenVPN clients in cipher-negotiation.rst

 - Explain the IV_NCP=2 client situation in 2.4 a bit better.
 - Make more clear what exact versions are meant in the old client section
 - add a missing - in a heading

Thanks to Richard Bohnhomme for initial proof reading.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200812085412.19178-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20714.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>