Commenting on danakj's clang-tidy PRs, it would've been helpful to just
have a quick reference for older checks. So while I've been getting
comments for new things, go back and comment the ones that predate
adding per-check comments.
I did this mainly by running clang-tidy and seeing whether we could
re-enable them, thus also fixing the clang-tidy wrapper script.
Adding backticks to try to make it easier to scan.