Hide `doc(hidden)` items from auto suggestions

[EFanZh]

For example, rust-analyzer suggests using futures::FutureExt, which is not publicly documented: https://docs.rs/futures/0.3.12/futures/?search=FutureExt.

[jhpratt]

This could probably be lumped in with #824.

[jonas-schievink]

futures::FutureExt is publicly documented and not marked #[doc(hidden)], so it should appear in completions. However it does look like flyimport suggestions ignore #[doc(hidden)], even after #9715.

[EFanZh]

@jonas-schievink The source code says futures::FutureExt is marked #[doc(hidden)]: https://docs.rs/futures/0.3.16/src/futures/lib.rs.html#107-108.

[lnicola]

The re-export is hidden, not the trait.

[flodiebold]

Soo if the reexport is hidden, does that mean we don't want to import futures::FutureExt and should use futures::future::FutureExt? 🤔 And we shouldn't autocomplete FutureExt when the user writes futures::?

[lnicola]

That's what I would expect.

[flodiebold]

What's the intention of these reexports then?

[flodiebold]

This example imports futures::StreamExt, for example.