Change server instance to cluster for UUID docs

[andersjanmyr]

Overview

Server instance is easily misunderstood. It can be interpreted as if there should be a unique UUID per node.

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches