Docs: clarify that Terraform state files are not include in backup rake task

Problem to solve

In a recent support ticket a customer informed us that in our documentation we do not specifically mention that Terraform state files are not included in the rake task that backs up GitLab.

Proposal

Add this information to our documentation on this rake task so that future customers aren't confused regarding whether or not Terraform state files are included in the backup rake task.

Other links/references

• Zendesk ticket (GitLab Internal)