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>