Update Output values docs.

website/docs/configuration/outputs.html.md

@@ -52,6 +52,9 @@ refers to the `private_ip` attribute exposed by an `aws_instance` resource
 defined elsewhere in this module (not shown). Any valid expression is allowed
 as an output value.
 
+-> **Note:** Outputs are only rendered once terraform applies changes to your
+infrastructure. Issue a `terraform plan` against your code won't render those.
+
 ## Accessing Child Module Outputs
 
 In a parent module, outputs of child modules are available in expressions as