Auto merge of #41791 - Mark-Simulacrum:doc-guidelines, r=frewsxcv

Auto merge of #41791 - Mark-Simulacrum:doc-guidelines, r=frewsxcv

Minor cleanup of UX guidelines.

I think this fixes https://github.com/rust-lang/rust/issues/34808. It covers the [long error code explanations normalization] by linking to the RFC, and cleaning up the list where long diagnostics are defined. While the [error message overhaul] isn't covered directly, I'm not really sure that more than the [existing section] on the error/warning/help messages is warranted; the overhaul linked didn't really specify any new guidelines, primarily just changing the output format.

[Long error code explanations normalization]: https://github.com/rust-lang/rfcs/blob/master/text/1567-long-error-codes-explanation-normalization.md
[Error message overhaul]: https://github.com/rust-lang/rust/issues/33240
[existing section]: https://github.com/rust-lang/rust/blob/master/src/doc/rustc-ux-guidelines.md#error-warning-help-note-messages