Document the --auth-token option

Document the --auth-token option

This isn't an option to be used directly in any configuration files,
but to be used via --client-connect scripts or --plugin making use of
OPENVPN_PLUGIN_CLIENT_CONNECT or OPENVPN_PLUGIN_CLIENT_CONNECT_V2.

 [v2 - Added lacking .B styling of options
     - Clarified the token life time ]

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1474118415-14666-1-git-send-email-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12506.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f8a367f7c51af5482013fa3d783cade376b047ed)