Updating downward-api/index.md for spec.nodeName

There are a couple awesome enhancements that would benefit from being documented here:

1. `spec.nodeName` for obtaining the host name
2. `spec.serviceAccountName` for obtaining the name of the service account the pod is running under

Right now these enhancements are documented only in a couple of issues and pull requests:

1. https://github.com/kubernetes/kubernetes/pull/27880
2. https://github.com/kubernetes/kubernetes/issues/24657
3. https://github.com/kubernetes/kubernetes/issues/21317

I have verified the functionality in the latest codebase:

1. https://github.com/kubernetes/kubernetes/blob/master/pkg/api/v1/conversion.go#L200

Also updating downward-api/dapi-pod.yaml to add a concrete example.

docs/user-guide/downward-api/dapi-pod.yaml

@@ -8,6 +8,10 @@ spec:
       image: gcr.io/google_containers/busybox
       command: [ "/bin/sh", "-c", "env" ]
       env:
+        - name: MY_NODE_NAME
+          valueFrom:
+            fieldRef:
+              fieldPath: spec.nodeName
         - name: MY_POD_NAME
           valueFrom:
             fieldRef:
@@ -20,4 +24,8 @@ spec:
           valueFrom:
             fieldRef:
               fieldPath: status.podIP
+        - name: MY_POD_SERVICE_ACCOUNT
+          valueFrom:
+            fieldRef:
+              fieldPath: spec.serviceAccountName
   restartPolicy: Never