Final answer:
A playbook is the document type that outlines the authority of a CSIRT in responding to a security incident.
Step-by-step explanation:
The document type that would outline the authority of a CSIRT in responding to a security incident is a Playbook. A playbook is a set of instructions and guidelines that outline the specific steps and actions to be taken during a security incident response. Playbooks provide the necessary guidance and authority for the CSIRT to effectively and efficiently respond to and mitigate security incidents.