rustdoc: Test & document `test_harness` code block attribute They were fully untested and undocumented previously despite being stable. Context: [#t-rustdoc > `test_harness` langstr @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/.60test_harness.60.20langstr/near/546748038). While at it, I've also improved the documentation for code block attributes in general. I was only inspired to do so because there was no good place for `test_harness` and because it got hard to skim due to a lack of subsubsections.