Document that `extern crate test` is still required for benchmarks with 2018 edition

[dekellum]

Originally reported in rust-lang/rust#55133, and of late it seems I'm not the only one surprised by the continued need to use extern crate test in each benchmark as a special case (and with edition="2018")

Currently the guide's Path clarity section doesn't seem to indicate there are any exceptions:

https://rust-lang-nursery.github.io/edition-guide/rust-2018/module-system/path-clarity.html

Here's a brief summary:

• extern crate is no longer needed

Wouldn't it be helpful to add this or other exceptional cases as a footnote or under the More details: No more extern crate section, with some rationale as indicated in rust-lang/rust#55133? My attempt at a user-centric elaboration:

#![feature(test)] is unstable and doesn't by itself import the test pseudo-crate for benchmarks.

Cc @Mark-Simulacrum (not sure if you also disagree with documenting this discrepancy)

[steveklabnik]

this isn't specific to test, it's for any of the crates in the sysroot, like proc_macro

and we should document this

[dekellum]

That sounds like a much better explanation, thanks.