DOC: explain changelog in devdocs

- changelog needs the number of the PR not the issue
- we don't changelog docs/test changes or any internal restructure that has no effect to the users (not a bugfix or a performance improvement)
- we don't changelog follow-up PRs of new features if they are not yet released ("affect-dev"), but may amend the original changelog entry with the number of the new PR.

Start in the devdocs changelog guidance and review the three rules stated in the issue: use the PR number, omit non-user-facing changes, and handle unreleased feature follow-ups appropriately. Done means the devdocs clearly explain these cases and match the wording and examples expected by the project.

Written by the indexing model from the issue text.