Improvements to terraform state storage setup

Two minor changes:

- Makes the administrative requirement before use of feature clear, by
separating out its sentence in the user doc.
- Adds a line about configuring terraform
storage settings for helm chart installations, in the admin doc.

doc/administration/terraform_state.md

@@ -18,6 +18,8 @@ The storage location of these files defaults to:
 
 These locations can be configured using the options described below.
 
+Use [external object storage](https://docs.gitlab.com/charts/advanced/external-object-storage/#lfs-artifacts-uploads-packages-external-diffs-pseudonymizer-terraform-state-dependency-proxy) configuration for [GitLab Helm chart](https://docs.gitlab.com/charts/) installations.
+
 ## Using local storage
 
 The default configuration uses local storage. To change the location where

doc/user/infrastructure/terraform_state.md

@@ -16,14 +16,16 @@ to securely store the state files in local storage (the default) or
 
 The GitLab managed Terraform state backend can store your Terraform state easily and
 securely, and spares you from setting up additional remote resources like
-Amazon S3 or Google Cloud Storage. After the GitLab administrator
-[sets it up](../../administration/terraform_state.md), its features include:
+Amazon S3 or Google Cloud Storage. Its features include:
 
 - Versioning of Terraform state files.
 - Supporting encryption of the state file both in transit and at rest.
 - Locking and unlocking state.
 - Remote Terraform plan and apply execution.
 
+A GitLab **administrator** must [setup the Terraform state storage configuration](../../administration/terraform_state.md)
+before using this feature.
+
 ## Permissions for using Terraform
 
 In GitLab version 13.1, [Maintainer access](../permissions.md) was required to use a