Document how to define LokiStack CR for memberlist.instanceAddrType.podIP

Before https://issues.redhat.com/browse/LOG-4008 (Logging 5.6.5) and https://issues.redhat.com/browse/LOG-3721 (Logging 5.7.0) if it was used a no private IP for pods (clusterNetwork), then, it was failing to start the Loki pods.

The fix enables to be able to use this no Private IPs for pods, but it's needed to modify the Logging as defined in article https://access.redhat.com/solutions/6998362 where it's needed to add:
```
apiVersion: loki.grafana.com/v1
kind: LokiStack
metadata:
  name: logging-loki
  namespace: openshift-logging
spec:
...
  hashRing: <--- this and the next lines
    type: memberlist
    memberlist:
      instanceAddrType: podIP
```

or with:
```
$ oc patch LokiStack logging-loki -n openshift-logging  --type=merge -p '{"spec": {"hashRing":{"memberlist":{"instanceAddrType":"podIP","type": "memberlist"}}}}'
```

This should be documentated for when installing Loki and how the Loki definition should be in case that using no private IPs