Explaining edge case behavior is one use-case for comments, and not the most valuable one in my estimation. Aside from that, often incorrect edge case handling is in both the code and the unit test because the problem is that the developer didn't understand the requirements. In my experience, in an undocumented and difficult codebase the tests will be as mysterious or unreliable as the code itself, which makes sense since they are usually written by the same people.
You'd be surprised. There are generations of programmers now who think nothing of writing 20+ unit tests with quite clear names demonstrating what the behaviour should be under a variety of conditions, but with virtually no other documentation. Especially true in dev shops that have high coverage requirements for a successful ci build.
