Docs for pod affinity namespace selector and pod affinity corss-namespace quota scope.

content/en/docs/concepts/scheduling-eviction/assign-pod-node.md

@@ -271,6 +271,18 @@ If omitted or empty, it defaults to the namespace of the pod where the affinity/
 All `matchExpressions` associated with `requiredDuringSchedulingIgnoredDuringExecution` affinity and anti-affinity
 must be satisfied for the pod to be scheduled onto a node.
 
+#### Namespace selector
+{{< feature-state for_k8s_version="v1.21" state="alpha" >}}
+
+Users can also select matching namespaces using `namespaceSelector`, which is a label query over the set of namespaces.
+The affinity term is applied to the union of the namespaces selected by `namespaceSelector` and the ones listed in the `namespaces` field.
+Note that an empty `namespaceSelector` ({}) matches all namespaces, while a null or empty `namespaces` list and 
+null `namespaceSelector` means "this pod's namespace".
+
+This feature is alpha and disabled by default. You can enable it by setting the
+[feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
+`PodAffinityNamespaceSelector` in both kube-apiserver and kube-scheduler.
+
 #### More Practical Use-cases
 
 Interpod Affinity and AntiAffinity can be even more useful when they are used with higher