Add documentation backreferences.

Author: efritz

README.md

@@ -14,14 +14,14 @@ This example uses the [root module](https://registry.terraform.io/modules/source
 
 The following variables must be supplied:
 
+**Note**: these variables must be set in tandem with changes to the target [Sourcegraph deployment](https://docs.sourcegraph.com/admin/deploy_executors).
+
 - `executor_sourcegraph_external_url`: The URL from which the target Sourcegraph instance is accessible from the executor instances.
 - `executor_sourcegraph_executor_proxy_username`: The shared executor username defined on the Sourcegraph deployment.
-- `executor_sourcegraph_executor_proxy_password`: The shared executor password defined on the Sourcegraph deployment.
+- `executor_sourcegraph_executor_proxy_password`: The shared executor password defined on the Sourcegraph deployment. The username and password values must be the same as the `EXECUTOR_FRONTEND_USERNAME` and `FRONTEND_USERNAME_PASSWORD` environment variables described in [Configuring executors and instance communication](https://docs.sourcegraph.com/admin/deploy_executors#configuring-executors-and-instance-communication).
 - `executor_queue_name`: The name of the target queue to process (e.g., `codeintel`, `batches`).
-- `executor_metrics_environment_label`: The name of the target environment (e.g., `staging`, `prod`).
-- `executor_instance_tag`: Compute instances are tagged by this value by the key `executor_tag`. We recommend this value take the form `{executor_queue_name}-{executor_metrics_environment_label}`.
-
-<!-- TODO(efritz) - reference product docs here when they exist -->
+- `executor_metrics_environment_label`: The name of the target environment (e.g., `staging`, `prod`). This value must be the same as the `EXECUTOR_METRIC_ENVIRONMENT_LABEL` environment variable as described in [Configuring auto scaling](https://docs.sourcegraph.com/admin/deploy_executors#aws).
+- `executor_instance_tag`: Compute instances are tagged by this value by the key `executor_tag`. We recommend this value take the form `{executor_queue_name}-{executor_metrics_environment_label}`. This value must be the same as `INSTANCE_TAG` as described in [Configuring observability](https://docs.sourcegraph.com/admin/deploy_executors#aws-1).
 
 All of this module's variables are defined in [variables.tf](./variables.tf).