docs: add note on updating terraform config after gcp filestore restore

docs/howto/filesystem-backups/restore-filesystem.md

@@ -17,16 +17,32 @@ Due to the length of the steps listed in that document, we will not repeat them
 ## GCP
 
 ```{note}
-We follow GCP's guidance for [restoring fileshares from a backup](https://cloud.google.com/filestore/docs/backup-restore#restore)
+We follow GCP's guidance for [restoring fileshares from a backup](https://cloud.google.com/filestore/docs/backup-restore#restore).
 ```
 
-To restore a share on a Filestore instance on GCP, we follow the documentation
-linked above. In short, this involves:
+To restore a share on a Filestore instance on GCP, we start by following the
+documentation linked above. In short, this involves:
 
 1. [Go to the Filestore instances page](https://console.cloud.google.com/filestore/instances) in the GCP console
-1. Click the instance ID of the Filestore you want to restore and click the "Backups" tab
-1. Locate the backup you want to restore from (most likely the most recently created), and click (...) "More actions"
-1. Click "Restore backup" and then select "Source instance"
-1. Click "Restore" and complete the dialog box that appears
+2. Click the instance ID of the Filestore you want to restore and click the "Backups" tab
+3. Locate the backup you want to restore from (most likely the most recently created), and click (...) "More actions"
+4. Click "Restore backup" and then select "Source instance"
+5. Click "Restore" and complete the dialog box that appears
 
-This should successfully restore the Filestore instance to its last backed-up state
+This should successfully restore the Filestore instance to its last backed-up state.
+
+Once this is done, you should also set the terraform variable `source_backup` to
+reference this web console change. Practically, you should configure something
+like below in the `terraform/gcp/projects/$CLUSTER_NAME.tfvars` file:
+
+```
+filestores = {
+  "filestore" : {
+    # ...
+    source_backup : "projects/<gcp project name>/locations/<filestore region>/backups/<backup name>",
+  },
+}
+```
+
+You can determine the value of `source_backup` by trying a `terraform plan`
+which should present the value in an associated error.