If the codebase is associated with a reasonably stable bug-tracker, I'd also put a bug # or a URL in the source comment if it's something nonobvious that is important for future maintainers to be aware of, but considered too long to document inline (if it's purely of historical interest, then yeah, leave it to the VCS history). Or a link to an archived mailing list discussion, or a CVE id, or some kind of hint.