explain effect of --topology subnet on --ifconfig

explain effect of --topology subnet on --ifconfig

The fact that the second parameter of --ifconfig is no longer
a "remote address" but a "netmask" when using --dev tun and
--topology subnet was not documented clearly enough.

Trac #370

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1430216419-11943-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9616
(cherry picked from commit 3a840739e43acc5ea15814be08debb9dbb7ba67c)