Investigate how modified changelogs work in release process

When a changelog is created by the tooling and then is modified by the user before usage in a release, the changes are not include/brought back to the Github Release/"cherry-pick".

This issue was brought up for discussion in https://github.com/angular/dev-infra/pull/260 but is being considered in a separate scope.

Start with the release tooling and the discussion in pull request 260. Trace how a tooling-created changelog is handled after a user edits it, and document what must change so those edits reach the GitHub Release or cherry-pick.

Written by the indexing model from the issue text.