Release note depend of your organization.
I can talk for my organization. We use release note in PDF format and every time we publish a clickonce or a backend version. We send to office manager the Release note. This is a document used by the top administrator of the business (not only IT). This document is a way for them to know what is going on. What have changed, new features that are now in production, bugs fixed, and other thing that they might want to explain to their user.
This is a document that can be between 3 to 4 pages, describing the job that has been done in this version with brief words.
