Textual fixes for Changes.rst

Textual fixes for Changes.rst

We will likely refer many people to the Changes.rst file once we've
released 2.4.  This commits tries to polish the language a bit, and
adds two real changes:

 - Remove duplicate mention of the changes --tls-cipher defaults

 - Move the 'redirect-gateway' behavioural change from 'features' to
   'behavioural changes'.

v2 - On the fly commit changes, based on comments from Selva Nair.
     DS also added a few minor corrections on top of that.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1482779743-9548-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13732.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit f38942d1440575e23d9f8713db435b434381486e)