docs: Further enhance the documentation related to SWEET32

docs: Further enhance the documentation related to SWEET32

The git master/2.4 code lacked some useful information about
the changes to --reneg-bytes, SWEET32 and weak ciphers (less
than 128-bits cipher blocks)

v2 - Fixed a couple of grammar/typo issues

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <1482509264-24550-1-git-send-email-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13682.html