man: Add missing --server-ipv6

man: Add missing --server-ipv6

During the conversion from .8 to .rst and further reorganizing of the
content into separate files, the --server-ipv6 entry got lost.  This
resurrects it again.

Trac: #1324

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200911154259.13837-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20970.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 8120e1ad4f20d4ec1fa9e191f3b15740c4aec448)