Audit our error messages for the new markdown check #3882
Description
checkMarkdown.ts now has a few new checks and merges others we used to have into one. A downside of this is our error messages are less descriptive than before to avoid them being too noisy. For example, look at #3870.
In this issue, read our README and the error message in checkMarkdown.ts to ensure we're giving adequate instructions to content writers.