fix based on reviews

Author: sanposhiho

keps/sig-autoscaling/1610-container-resource-autoscaling/README.md

@@ -492,13 +492,16 @@ https://storage.googleapis.com/k8s-triage/index.html
 We expect no non-infra related flakes in the last month as a GA graduation criteria.
 -->
 
-- `/test/e2e/autoscaling/horizontal_pod_autoscaling.go`: https://storage.googleapis.com/k8s-triage/index.html?sig=autoscaling&job=ci-kubernetes-e2e-gci-gce-autoscaling&test=%5C(Container%20Resource%5C)
+**k8s-triage**
+
+- https://storage.googleapis.com/k8s-triage/index.html?sig=autoscaling&job=ci-kubernetes-e2e-gci-gce-autoscaling&test=Container%20Resource
 
 **tests**
 
 - https://github.com/kubernetes/kubernetes/blob/d4750857760ae55802f69989dc2451feeb9a29e5/test/e2e/autoscaling/horizontal_pod_autoscaling.go#L61
 - https://github.com/kubernetes/kubernetes/blob/d4750857760ae55802f69989dc2451feeb9a29e5/test/e2e/autoscaling/horizontal_pod_autoscaling.go#L163
 - https://github.com/kubernetes/kubernetes/blob/d4750857760ae55802f69989dc2451feeb9a29e5/test/e2e/autoscaling/horizontal_pod_autoscaling.go#L120
+- https://github.com/kubernetes/kubernetes/blob/d4750857760ae55802f69989dc2451feeb9a29e5/test/e2e/autoscaling/custom_metrics_stackdriver_autoscaling.go#L323
 
 ### Graduation Criteria
 
@@ -699,13 +702,20 @@ NOTE: Also set `disable-supported` to `true` or `false` in `kep.yaml`.
 
 The feature can be disabled in Alpha and Beta versions
 by restarting kube-apiserver and kube-controller-manager with the feature-gate off.
+
+As described in [Upgrade / Downgrade Strategy](#upgrade-/-downgrade-strategy),
+during the feature-gate off, all existing `ContainerResource` will be ignored by the HPA controller.
+
 In terms of Stable versions, users can choose to opt-out by not setting the
 `ContainerResource` type metric in their HPA.
 
 ###### What happens if we reenable the feature if it was previously rolled back?
 
 HPA with `ContainerResource` type metric can be created and can be handled by HPA controller.
 
+If there have been HPAs with the `ContainerResource` type metric created before the roll back,
+those `ContainerResource` will be restarted to get handled by the HPA controller.
+
 ###### Are there any tests for feature enablement/disablement?
 
 <!--
@@ -755,7 +765,7 @@ What signals should users be paying attention to when the feature is young
 that might indicate a serious problem?
 -->
 
-N/A
+- so many HPAs are in `ScalingActive: false` condition with `FailedGetContainerResourceMetric` reason.
 
 ###### Were upgrade and rollback tested? Was the upgrade->downgrade->upgrade path tested?
 
@@ -807,8 +817,8 @@ Recall that end users cannot usually observe component logs or access metrics.
 -->
 
 - [x] Events
-  - `SuccessfulRescale` event with `memory resource utilization (percentage of request) above target`
-    - Note that we cannot know if this reason is due to the `Resource` metric or `ContainerResource` in the current implementation. It'll be fixed to be able to distinguish.
+  - `SuccessfulRescale` event with `memory/cpu/etc resource utilization (percentage of request) above/below target`
+    - Note that we cannot know if this reason is due to the `Resource` metric or `ContainerResource` in the current implementation. We'll change this reason for `ContainerResource` to `memory/cpu/etc container resource utilization (percentage of request) above/below target` so that we can distinguish.
 - [x] API .status
   - When something wrong with the container metrics, `ScalingActive` condition will be false with `FailedGetContainerResourceMetric` reason.
 
@@ -1000,7 +1010,11 @@ For each of them, fill in the following information by copying the below templat
     - Testing: Are there any tests for failure mode? If not, describe why.
 -->
 
-N/A
+- Failed to get container resource metric.
+  - Detection:  `ScalingActive: false` condition with `FailedGetContainerResourceMetric` reason.
+  - Mitigations: remove failed `ContainerResource` in HPAs.
+  - Diagnostics: Related errors should be printed as the messages of `ScalingActive: false`. 
+  - Testing: https://github.com/kubernetes/kubernetes/blob/0e3818e02760afa8ed0bea74c6973f605ca4683c/pkg/controller/podautoscaler/replica_calculator_test.go#L451
 
 ###### What steps should be taken if SLOs are not being met to determine the problem?
 
@@ -1023,6 +1037,11 @@ not need to be as detailed as the proposal, but should include enough
 information to express the idea and why it was not acceptable.
 -->
 
+There's an alternative way to scale on container-level metrics without introducing ContainerResource metrics. 
+
+Users can export resource consumption metrics from containers on their own to an external metrics source and then configure HPA based on this external metric. 
+However this is cumbersome and results in delayed scaling decisions as using the external metrics path typically adds latency compared to in-cluster resource metrics path.
+
 ## Infrastructure Needed (Optional)
 
 <!--