Update auto-deploy-image version in docs and note on backups

doc/topics/autodevops/upgrading_auto_deploy_dependencies.md

@@ -100,9 +100,15 @@ If your Auto DevOps project has an active environment that was deployed with the
      MIGRATE_HELM_2TO3: "true"
 
    .auto-deploy:
-     image: registry.gitlab.com/gitlab-org/cluster-integration/auto-deploy-image:v2.0.0-beta.1
+     # Optional: If you are on GitLab 13.12 or older, pin the auto-deploy-image
+     # image: registry.gitlab.com/gitlab-org/cluster-integration/auto-deploy-image:v2.6.0
      variables:
        AUTO_DEVOPS_FORCE_DEPLOY_V2: 1
+       # If you have non-public pipelines, you can back up the entire namespace in a job artifact
+       # prior to the migration by setting the CI variable BACKUP_NAMESPACE to a non-empty value.
+       # WARNING: If you have public pipelines, this artifact will be public and can
+       #          expose your secrets.
+       # BACKUP_HELM2_RELEASES: 1
    ```
 
 1. Run the `<environment-name>:helm-2to3:migrate` job.
@@ -110,10 +116,14 @@ If your Auto DevOps project has an active environment that was deployed with the
 1. If the deployment succeeds, you can safely run `environment:helm-2to3:cleanup`.
    This deletes all Helm 2 release data from the namespace.
 
-   If you accidentally delete the Helm 2 releases before you are ready, the `<environment-name>:helm2to3:migrate`
+   If you set `BACKUP_HELM2_RELEASES` to a non-empty value, then the 
+   the `<environment-name>:helm2to3:migrate` job
    job saves a backup for 1 week in a job artifact called `helm-2-release-backups`.
-   The backup is in a Kubernetes manifest file that can be restored using
-   `kubectl apply -f $backup`.
+   If you accidentally delete the Helm 2 releases before you are ready, then
+   this backup is in a Kubernetes manifest file that can be restored using
+   `kubectl apply -f $backup`. 
+   **WARNING:** This artifact can contain secrets and will be visible to any
+   user who can see your job. 
 1. Remove the `MIGRATE_HELM_2TO3` CI/CD variable.
 
 #### In-Cluster PostgreSQL Channel 2