Updated Doc: Omitting AWS access key and ID key=>value pairs when using AWS 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.

/cc @stanhu

See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13871 & https://github.com/fog/fog-aws/issues/162#issuecomment-325427231