Incorporate guidance on long error messages

[nikomatsakis]

We have

https://rust-lang.github.io/rfcs/1567-long-error-codes-explanation-normalization.html

which has some conventions for long error codes. This should be in the rustc-guide.

[nikomatsakis]

cc @GuillaumeGomez

[GuillaumeGomez]

@eddyb wanted to get rid of them and I've heard (for a while now, I'm starting to wonder if it'll ever happen...) that they'd adapt to users' code directly to make them even easier to understand. But I assume it should be in the rustc-guide nevertheless.

cc @rust-lang/docs

[nikomatsakis]

I think we should definitely incorporate the text for now.