Update custom_roles.md to include a warning about privilege escalation
See discussion in https://gitlab.slack.com/archives/CLM1D8QR0/p1699278064936439?thread_ts=1699276720.076919&cid=CLM1D8QR0 for additional context. This change is to inform developers contributing to custom roles to ensure we don't introduce privilge escalation by allowing a low access base role to create/manage an artifact as a high access role.