Everybody loves to talk about self-documenting code, self-documenting architecture, self-eplanatory everything.
If it's obvious you should obviously know it. If it's intuitive you should know it without actually knowing it.
This comes up a lot when you talk about documenting a codebase and it comes up a lot with refactoring.
Pull requests (contributions to the codebase) are fun because they also document a change of code from one version to another. How do you document the space between old and new?
It's not always easy to describe those changes - you can write a PR description or create meticulous commits and both are amazing, but they're not always enough.
Whenever I submit a pull-request I review it myself and comment on the lines I want to draw attention to. Either I'm justifying the approach or the change, or I'm asking for some help. Most of it is discarded, but some of it is kept.
Those comments are different because they were made on top of the code, not as a part of it. Like drawing notes on what you did after the fact.
Marginalia for code.