Hi Andy,
I am afrad that we do not currently have any release notes available for the Milestone functionality. This is work we are still actively delivering on and will first be releasing it as beta functionality for our SaaS customers. Once we have released this functionality and it has been removed from behind the feature toggle this functionality will be delivered to our On-Premises customers. At this time we don't have a definite timeframe for when tthat will be.
What I can tell you is that Milestones are named dates that allow you to track your work according to important deadlines for your business. They will be available on all artifact types (Stories, Defects, Portfolio Items, etc).
Please let me know if I can answer any further questions.
Kind Regards,
Kate