Outdated `--explain` text for E0432 "An import was unresolved."

[mattheww]

See E0432 for the current text.

It currently includes:
«
Paths in use statements are relative to the crate root. To import items relative to the current and parent modules, use the self:: and super:: prefixes, respectively.
»
The first sentence is untrue in Rust 2018, and so the advice about self:: is unhelpful.

It also says
«
Or, if you tried to use a module from an external crate, you may have missed the extern crate declaration (which is usually placed in the crate root):
»
which I think is unhelpful in Rust 2018.

[BookOwl]

I can confirm this is still present in Rust 1.41. I would be happy to make a PR to fix this if someone could give me a pointer on where to make the change.

[mattheww]

The text lives in src/librustc_error_codes/error_codes/E0432.md

https://github.com/rust-lang/rust/blob/master/src/librustc_error_codes/error_codes/E0432.md