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

doc/openvpn.8

@@ -798,6 +798,12 @@ driver supports an
 command which sets a subnet instead of a remote endpoint IP address.
 
 This option exists in OpenVPN 2.1 or higher.
+
+Note: Using
+.B \-\-topology subnet
+changes the interpretation of the arguments of
+.B \-\-ifconfig
+to mean "address netmask", no longer "local remote".
 .\"*********************************************************
 .TP
 .B \-\-tun-ipv6
@@ -861,16 +867,21 @@ May be used in order to execute OpenVPN in unprivileged environment.
 Set TUN/TAP adapter parameters. 
 .B l
 is the IP address of the local VPN endpoint.
-For TUN devices,
+For TUN devices in point-to-point mode,
 .B rn
 is the IP address of the remote VPN endpoint.
-For TAP devices,
+For TAP devices, or TUN devices used with
+.B \-\-topology subnet,
 .B rn
-is the subnet mask of the virtual ethernet segment
+is the subnet mask of the virtual network segment
 which is being created or connected to.
 
 For TUN devices, which facilitate virtual
-point-to-point IP connections,
+point-to-point IP connections (when used in
+.B \-\-topology net30
+or
+.B p2p
+mode),
 the proper usage of
 .B \-\-ifconfig
 is to use two private IP addresses
@@ -885,7 +896,9 @@ you will be pinging across the VPN.
 
 For TAP devices, which provide
 the ability to create virtual
-ethernet segments,
+ethernet segments, or TUN devices in
+.B --topology subnet
+mode (which create virtual "multipoint networks"),
 .B \-\-ifconfig
 is used to set an IP address and
 subnet mask just as a physical