Update user-guide/node-selection/index.md to include pod affinity/anti-affinity

and addd an example. Also clean up user-guide/node-selection/index.md a bit.

docs/user-guide/node-selection/pod-with-node-affinity.yaml

@@ -1,7 +1,7 @@
 apiVersion: v1
 kind: Pod
 metadata:
-  name: with-labels
+  name: with-node-affinity
   annotations:
     scheduler.alpha.kubernetes.io/affinity: >
       {
@@ -24,5 +24,5 @@ metadata:
     another-annotation-key: another-annotation-value
 spec:
   containers:
-  - name: with-labels
+  - name: with-node-affinity
     image: gcr.io/google_containers/pause:2.0