doc: cleanup for --data-ciphers and related

doc: cleanup for --data-ciphers and related

- Fix various formatting inconsistencies
- Remove outdated (as of 2.6) information from
  --data-ciphers and instead add a link to
  cipher negotiation chapter.
- Some drive-by fixes in related code comments
  and log messages as I was reading them.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220628080814.745-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24575.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>