In a post on his site Stefan Koopmanschap suggests something that, while not as commonly done as writing docblocks for the actual code, could help with a better understanding of the application's tests: writing docblocks on tests too.

If you've met me at any time in the previous 20 years and you discussed unit testing with me, chances are pretty big that I'd have told you that your test methods in your unit tests don't really need docblocks, because the test methods would be named in such a way that they were descriptive.

[...] Tests can be hard to real though, especially (but not exclusively) when testing legacy code. You may have lots of mocks to configure, for instance, or you may have several similar tests that are testing specific situations, edge cases or bugs you found along the way that you wanted to test before you them.

[...] Documentation is important. It lowers the bus factor, makes it easier to on-board new developers (or temporary external developers, waves) and makes you think about your own code in a different way. We're (getting?) used to documenting our software, but why not document our tests by giving it a bit more context?

He starts by talking about his process for testing (outline the desired tests then fill them in) and how adding docblocks helped him avoid a "nonsensical" test that wouldn't have provided much value. He then briefly discusses what would be useful to document in the test docblocks and provides some examples to illustrate. Finally he suggests something called "Docblock Driven Testing" where the docblock descriptions help to define what is to be tested and help to make more human-readable sense of what can be tested effectively.