Unit tests as documentation

One - unit tests explain nothing. They show what the output should be for a given input, but not why, or how you get there. I'm surprised by the nonchalant claim that "unit tests explain code". Am I missing something about the meaning of the english word "explain"?

Two - so any input value outside of those in unit tests is undocumented / unspecified behavior? A documentation can contain an explanation in words, like what relation should hold between the inputs and outputs in all cases. Unit tests by their nature can only enumerate a finite number of cases.

This seems like such an obviously not great idea...

Not sure about this, but I like it the way it is done in the Rust ecosystem.

In Rust, there are two types of comments. Regular ones (e.g. starting with //) and doc-comments (e.g. starting with ///). The latter will land in in the generated documentation when you run cargo doc.

And now the cool thing: If you have example code in these doc comments, e.g. to explain how a feature of your library can be used, that script will automatically become part of the tests per default. That means you are unlikey to forget to update these examples when your code changes and you can use them as tests at the same time by asserting something at the end (which also communicates the outcome to the reader).