That's what the commit message that added that test is for. And an example of why I'm strongly against squashing commits.

For a while now I've been dreaming about a system that lets me link tests to documentation. In a similar way that citations of other documents support claims in many domains, claims about software could be backed up by tests that demonstrate the claim. Then, when a test fails, the system could surface the relevant pieces of documentation (whether they're developer facing or user facing) so the developer knows the _why_ they're trying to preserve as they update some combination of the code, the test, and the documentation.

I'd guess you could get pretty far by just adding comments with links to product specs, JIRA tickets, or Slack/chat threads, as applicable.