improve note and revise idc config (#1904)

Co-authored-by: pingcap-github-bot <sre-bot@pingcap.com>

charts/tidb-operator/values.yaml

@@ -205,9 +205,11 @@ admissionWebhook:
   ## failurePolicy are applied to ValidatingWebhookConfiguration which affect tidb-admission-webhook
   ## refer to https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#failure-policy
   failurePolicy:
-    ## the validation webhook would check the request of the given resources, and the failurePolicy is recommended as Ignore
+    ## the validation webhook would check the request of the given resources.
+    ## If the kubernetes api-server version >= 1.15.0, we recommend the failurePolicy as Fail, otherwise, as Ignore.
     validation: Ignore
-    ## the mutation webhook would mutate the request of the given resources, and the failurePolicy is recommended as Ignore
+    ## the mutation webhook would mutate the request of the given resources.
+    ## If the kubernetes api-server version >= 1.15.0, we recommend the failurePolicy as Fail, otherwise, as Ignore.
     mutation: Ignore
   ## tidb-admission-webhook deployed as kubernetes apiservice server
   ## refer to https://github.com/openshift/generic-admission-server

ci/deploy_tidb_operator_staging.groovy

@@ -23,12 +23,16 @@ scheduler:
   replicas: 2
 admissionWebhook:
   create: true
+  replicas: 2
   validation:
     statefulSets: true
     pods: true
     pingcapResources: false
   mutation:
     pingcapResources: true
+  failurePolicy:
+    validation: Fail
+    mutation: Fail
 features:
   - AutoScaling=true
 '''