Extended the secrets part

Makes clear that secrets should be both created under k8s and
speficied in the yaml.

doc/user/project/clusters/serverless/index.md

@@ -431,7 +431,8 @@ The sample function can now be triggered from any HTTP client using a simple `PO
 
 ### Secrets
 
-To access your Kubernetes secrets from within your function, the secrets should be created under the namespace of your serverless deployment.
+To access your Kubernetes secrets from within your function, the secrets should be created under the namespace of your serverless deployment and specified in your `serverless.yml` file as above.
+You can create secrets in several ways. The following sections show some examples.
 
 #### CLI example