extern_prelude check is breaking rustdoc

[sdroege]

See rust-lang/futures-rs#1003 (comment)

 Documenting futures-channel v0.2.1 (file:///home/travis/build/rust-lang-nursery/futures-rs/futures-channel)
error[E0658]: access to extern crates through prelude is experimental (see issue #44660)
  |
  = help: add #![feature(extern_prelude)] to the crate attributes to enable
error: Could not document `futures-channel`.
Caused by:
  process didn't exit successfully: `rustdoc --crate-name futures_channel futures-channel/src/lib.rs -o /home/travis/build/rust-lang-nursery/futures-rs/target/doc --cfg feature="default" --cfg feature="futures-core" --cfg feature="std" -L dependency=/home/travis/build/rust-lang-nursery/futures-rs/target/debug/deps --extern futures_core=/home/travis/build/rust-lang-nursery/futures-rs/target/debug/deps/libfutures_core-c5293a3c307e0c7b.rmeta` (exit code: 101)
warning: build failed, waiting for other jobs to finish...

[Nemo157]

Here's a short summary of the issue, there's links to specific bits of source and the old log of rustdoc before extern_prelude made it fail in the linked comment above:

Have some documentation using the new [`Something`](somecrate::Something) doc links where somecrate is an extern crate but is not available in the current scope. Previously this would give a warning about it being unresolvable, now it will give an error that you must use the extern_prelude feature.

I think being able to use the extern_prelude feature in these doc links is great, it will really help with some cases where you want to refer to something in a context that doesn't actually use it. But when not using the feature doc links like these should block it from being suggested and instead treat it as a normal unresolvable path.