Update backup_restore.md documentation regarding S3 and IAM profiles.
What does this MR do?
Update configuration documentation regarding backup transfer to S3 when using IAM profiles.
gitlab-org/gitlab-ce@7f8ef19c most likely broke the old configuration format where empty string aws_access_key_id & aws_secret_access_key were still OK. From 9.5 onwards moving backups to S3 using IAM profiles will fail if they are configured.