Add documentation for PERSIST_TUN_ACTION (Android specific)

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1412712650-5173-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9090

Signed-off-by: Gert Doering <gert@greenie.muc.de>