In Discourse, you can make an edit message in case the reasons for your edit are unclear, or if it was a huge edit that would take a long time to review. (Applies to moderators only.)
Simply start your edit, make your changes, and voila there's an (i) button that, when you click, it shows a little text box where you can type your reasoning.
When you look at the edit history, it shows your edit message along with it.