Comment by __MatrixMan__

Often, tests are parameterized over lists of cases such that you can document the general case near the code and document the specific cases near each parameter. I've even seen test frameworks that consume an excel spreadsheet provided by product so that the test results are literally a function of the requirements.

Would we prefer better docs than some comments sprinkled in strategic places in test files? Yes. Is having them with the tests maybe the best we can do for a certain level of effort? Maybe.

If the alternative is an entirely standalone repository of docs which will probably not be up to date, I'll take the comments near the tests. (Although I don't think this approach lends itself to unit tests.)