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.)