document the nic link meaning in routed mode

Currently this feature is hypervisor specific, but under kvm we use it
by default (aka: if the user hasn't specified another network script
which overrides our auto-generated one) to specify the routing table.
This was already hinted as a possibility in the manpages, but never
explicitely stated as a reality, so we update gnt-cluster(8) to actually
say this, and gnt-{backup,instance}(8) to point to gnt-cluster(8) for
more information.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

man/gnt-backup.sgml

@@ -214,7 +214,7 @@
                   this NIC to, in routed mode it's intended to
                   differentiate between different routing tables/instance
                   groups (but the meaning is dependent on the network
-                  script)</simpara>
+                  script, see gnt-cluster(8) for more details)</simpara>
               </listitem>
             </varlistentry>
           </variablelist>

man/gnt-cluster.sgml

@@ -432,10 +432,11 @@
             <term>link</term>
             <listitem>
               <para>
-                In bridged mode the default NIC bridge. In routed mode
-                it represents an hypervisor-vif-script dependent value
-                to allow different instance groups (perhaps via multiple
-                routing tables)
+                In bridged mode the default NIC bridge. In routed mode it
+                represents an hypervisor-vif-script dependent value to allow
+                different instance groups. For example under the KVM default
+                network script it is interpreted as a routing table number or
+                name.
               </para>
             </listitem>
           </varlistentry>

man/gnt-instance.sgml

@@ -183,7 +183,7 @@
                   this NIC to, in routed mode it's intended to
                   differentiate between different routing tables/instance
                   groups (but the meaning is dependent on the network
-                  script)</simpara>
+                  script, see gnt-cluster(8) for more details)</simpara>
               </listitem>
             </varlistentry>
           </variablelist>