man: Improve the --keepalive section

man: Improve the --keepalive section

Just minor clarifications and corrections of the --keepalive option.

  v2 - Changed from ps/pto to interval/timeout
     - Rephrased the server-side timeout doubling parapgraph

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1478007489-17163-1-git-send-email-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12866.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>