Make it easier to generate update-description files

It should be a little simpler to generate update-description files, e.g. when acting as the release manager.

At least detailed documentation is a must. A sanity checking tool would be better. A fully automated generator would be best. Calling this a documentation task to start with.

Parent Task: #5490 (closed)

Original created by @intrigeri on 6446 (Redmine)