add note about import vs apply for project service resources (#4949)

Signed-off-by: Modular Magician <[email protected]>

website/docs/r/google_project_service.html.markdown

@@ -45,3 +45,5 @@ Project services can be imported using the `project_id` and `service`, e.g.
 ```
 $ terraform import google_project_service.my_project your-project-id/iam.googleapis.com
 ```
+
+Note that unlike other resources that fail if they already exist, `terraform apply` can be successfully used to re-enable already enabled services. This means that when importing existing resources into Terraform, you can either import the `google_project_service` resources or treat them as new infrastructure and run `terraform apply` to re-enable them and add them to state.