Enhance VM in multi-network meshes documentation

[sfudeus]

I recently tried to setup inbound mesh expansion with Istio 1.7, following the guide in https://istio.io/latest/docs/examples/virtual-machines/multi-network/#installing-the-istio-control-plane.

The one essential information, which is missing, is that the ingressgateway service must be of type LoadBalancer to allow for discovery of the gateway IP. E.g. externalIp is not sufficient. This minor information cost me several hours of debugging, trying and searching.
A pointer to the reference for GlobalMeshOptions (https://istio.io/latest/docs/reference/config/istio.mesh.v1alpha1/#Network-IstioNetworkGateway) would be helpful as well - esp. if there the part about the registryServiceName would be enhanced for the need for LoadBalancer service type as well.

Eventually I found the required hint e.g. here: https://github.com/istio/istio/blob/master/operator/cmd/mesh/testdata/manifest-generate/data-snapshot/charts/global.yaml#L315