update doc for BoundServiceAccountTokenVolume

content/en/docs/reference/access-authn-authz/service-accounts-admin.md

@@ -10,14 +10,15 @@ weight: 50
 ---
 
 <!-- overview -->
+
 This is a Cluster Administrator guide to service accounts. You should be familiar with
 [configuring Kubernetes service accounts](/docs/tasks/configure-pod-container/configure-service-account/).
 
 Support for authorization and user accounts is planned but incomplete. Sometimes
 incomplete features are referred to in order to better describe service accounts.
 
-
 <!-- body -->
+
 ## User accounts versus service accounts
 
 Kubernetes distinguishes between the concept of a user account and a service account
@@ -55,35 +56,49 @@ It acts synchronously to modify pods as they are created or updated. When this p
 
 1. If the pod does not have a `ServiceAccount` set, it sets the `ServiceAccount` to `default`.
 1. It ensures that the `ServiceAccount` referenced by the pod exists, and otherwise rejects it.
+1. It adds a `volume` to the pod which contains a token for API access if neither the ServiceAccount `automountServiceAccountToken` nor the Pod's `automountServiceAccountToken` is set to `false`.
+1. It adds a `volumeSource` to each container of the pod mounted at `/var/run/secrets/kubernetes.io/serviceaccount`, if the previous step has created a volume for ServiceAccount token.
 1. If the pod does not contain any `ImagePullSecrets`, then `ImagePullSecrets` of the `ServiceAccount` are added to the pod.
-  1. It adds a `volume` to the pod which contains a token for API access.
-  1. It adds a `volumeSource` to each container of the pod mounted at `/var/run/secrets/kubernetes.io/serviceaccount`.
 
 #### Bound Service Account Token Volume
+
 {{< feature-state for_k8s_version="v1.21" state="beta" >}}
 
 When the `BoundServiceAccountTokenVolume` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/) is enabled, the service account admission controller will
-add a projected service account token volume instead of a secret volume. The service account token will expire after 1 hour by default or the pod is deleted. See more details about [projected volume](/docs/tasks/configure-pod-container/configure-projected-volume-storage/).
+add the following projected volume instead of a secret volume for the non-expiring service account token created by Token Controller.
 
-This feature depends on the `RootCAConfigMap` feature gate enabled which publish a "kube-root-ca.crt" ConfigMap to every namespace. This ConfigMap contains a CA bundle used for verifying connections to the kube-apiserver.
-1. If the pod does not have a `serviceAccountName` set, it sets the
-   `serviceAccountName` to `default`.
-1. It ensures that the `serviceAccountName` referenced by the pod exists, and
-   otherwise rejects it.
-1. If the pod does not contain any `imagePullSecrets`, then `imagePullSecrets`
-   of the ServiceAccount referenced by `serviceAccountName` are added to the pod.
-1. It adds a `volume` to the pod which contains a token for API access
-   if neither the ServiceAccount `automountServiceAccountToken` nor the Pod's
-   `automountServiceAccountToken` is set to `false`.
-1. It adds a `volumeSource` to each container of the pod mounted at
-   `/var/run/secrets/kubernetes.io/serviceaccount`, if the previous step has
-   created a volume for ServiceAccount token.
+```yaml
+- name: kube-api-access-c5cs8
+  projected:
+    defaultMode: 420 # 0644
+    sources:
+      - serviceAccountToken:
+          expirationSeconds: 3600
+          path: token
+      - configMap:
+          items:
+            - key: ca.crt
+              path: ca.crt
+          name: kube-root-ca.crt
+      - downwardAPI:
+          items:
+            - fieldRef:
+                apiVersion: v1
+                fieldPath: metadata.namespace
+              path: namespace
+```
 
-You can migrate a service account volume to a projected volume when
-the `BoundServiceAccountTokenVolume` feature gate is enabled.
-The service account token will expire after 1 hour or the pod is deleted. See
-more details about
-[projected volume](/docs/tasks/configure-pod-container/configure-projected-volume-storage/).
+This projected volume consists of three sources:
+
+1. A ServiceAccountToken acquired from kube-apiserver via TokenRequest API. It will expire after 1 hour by default or the pod is deleted. It is bound to the pod and has kube-apiserver as the audience.
+1. A ConfigMap contains a CA bundle used for verifying connections to the kube-apiserver. This feature depends on the `RootCAConfigMap` feature gate enabled which publish a "kube-root-ca.crt" ConfigMap to every namespace.
+1. A DownwardAPI that references the namespace of the pod.
+
+See more details about [projected volume](/docs/tasks/configure-pod-container/configure-projected-volume-storage/).
+
+You can manually migrate a secret-based service account volume to a projected volume when
+the `BoundServiceAccountTokenVolume` feature gate is not enabled by adding the above
+projected volume to the pod spec. However, `RootCAConfigMap` needs to be enabled.
 
 ### Token Controller