Add base64 encoded data configuration (#11793)

for the issue [11774](https://github.com/kubernetes/website/issues/11774)

content/en/docs/tasks/access-application-cluster/configure-access-multiple-clusters.md

@@ -140,6 +140,14 @@ users:
     username: exp
 ```
 
+The `fake-ca-file`, `fake-cert-file` and `fake-key-file` above is the placeholders
+for the real path of the certification files. You need change these to the real path
+of certification files in your environment.
+
+Some times you may want to use base64 encoded data here instead of the path of the 
+certification files, then you need add the suffix `-data` to the keys. For example,
+`certificate-authority-data`, `client-certificate-data`, `client-key-data`.
+
 Each context is a triple (cluster, user, namespace). For example, the
 `dev-frontend` context says, Use the credentials of the `developer`
 user to access the `frontend` namespace of the `development` cluster.