Rollup merge of #136124 - adetaylor:test-comment, r=compiler-errors

Arbitrary self types v2: explain test.

The purpose of this test wasn't obvious, as ```@traviscross``` noted. Add a comment.

Confession: although this test was added to demonstrate this particular corner-case, I can no longer reproduce the original problem, even if I adjust `rustc` to do the "wrong" thing. I have spent several hours trying to adjust the case to trigger the "faulty" behavior with no success. This test may therefore not be as useful as it originally was. But it still seems worthwhile retaining as a regression test that we don't break things in these quirky circumstances. Ideally we'd find a new test which tests this behavior but I've failed to come up with one.

r? ```@traviscross```

Author: matthiaskrgr