rustdoc: Test & document test_harness code block attribute

[fmease]

They were fully untested and undocumented previously despite being stable.
Context: #t-rustdoc > `test_harness` langstr @ 💬.

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.

[rustbot]

r? @GuillaumeGomez

rustbot has assigned @GuillaumeGomez.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[fmease]

It's not any old ENBF¹ (variant), it's ABNF² specifically (from IETF RFC 5234) which I previously didn't know about (and thus was confused about the syntax).

Footnotes

1. Extended Backus-Naur form. ↩

2. Augmented Backus-Naur form. ↩

[fmease]

Prose terminals are to be denoted by <…> not by (…) (which is grouping) per IETF RFC 5234 (Ctrl+F for prose-val).

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

To only update this specific test, also pass `--test-args doctest/test-harness.rs`

error: 1 errors occurred comparing output.
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="0" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustdoc" "/checkout/tests/rustdoc-ui/doctest/test-harness.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/rustdoc-ui/doctest/test-harness" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Cdebuginfo=0" "--test" "--test-args" "--test-threads=1"
--- stdout -------------------------------

running 4 tests
test /checkout/tests/rustdoc-ui/doctest/test-harness.rs - (line 14) ... FAILED
test /checkout/tests/rustdoc-ui/doctest/test-harness.rs - (line 25) ... FAILED
---
test group::element ... FAILED

failures:

---- group::element stdout ----

thread 'group::element' (321554) panicked at /checkout/tests/rustdoc-ui/doctest/test-harness.rs:6:9:
assertion failed: false
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
---
---- /checkout/tests/rustdoc-ui/doctest/test-harness.rs - (line 34) stdout ----
error: non-item macro in item position: assert
##[error]  --> /checkout/tests/rustdoc-ui/doctest/test-harness.rs:35:1
   |
LL | assert!(true);
   | ^^^^^^^^^^^^^

error: aborting due to 1 previous error

Couldn't compile the test.