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/index.md

@@ -22,9 +22,11 @@ the Pod's name, for example, and inject it into this well-known variable.
 
 The following information is available to a `Pod` through the downward API:
 
-*   The pod's name
+*   The node's name 
+*   The pod's name
 *   The pod's namespace
 *   The pod's IP
+*   The pod's service account name 
 *   A container's cpu limit
 *   A container's cpu request
 *   A container's memory limit
@@ -101,10 +103,12 @@ In future, it will be possible to specify an output format option.
 
 Downward API volumes can expose:
 
+*   The node's name
 *   The pod's name
 *   The pod's namespace
 *   The pod's labels
 *   The pod's annotations
+*   The pod's service account name 
 *   A container's cpu limit
 *   A container's cpu request
 *   A container's memory limit