In the case it is or it really needs an explanation it stays in the codebase, otherwise you take it, e.g. to GitHub issues or PR comments.
Another thing that's easy, but often overlooked: putting relevant URLs in comments. The best comments explain why some approach was taken; linking to a discussion on JIRA, or the Stack Overflow answer we c̶o̶p̶y̶-̶p̶a̶s̶t̶e̶d̶ ̶f̶r̶o̶m̶ used as inspiration, is (a) quicker than writing a whole explanation, and (b) gives better evidence of what's going on (e.g. that the CTO explicitly asked for this-or-that; or that the SO question now has a better answer; etc.)
If I want to know why something was done, I expect to be able to use git log or git blame on the file or line and get find a commit message, issue tracker link or pull request reference that I can pursue.
Sometimes even longer :)
is my all time favorite.