Permit attributes (esp. doc comments) inside use-trees

[stanislav-tkach]

The following is currently not permitted:

pub use third_party::{
    /// Doc-comment explaining reexported entity.
    some_function,
    // Normal comments are allowed, though.
    some_other_function,
};

I'm very fond of group usings, so I prefer to use them everywhere, but it is impossible in situations where the documentation is needed (we have encountered this problem here, by the way).

[Havvy]

I think this requires a dedicated RFC to outer attributes on that location.

[stanislav-tkach]

@Havvy Makes sense, but I'm not ready to take it on myself. I hope someone will be interested in writing an RFC.

[QuietMisdreavus]

We don't even display docs on use statements to begin with, so that would need to happen as well.

I don't think allowing attributes inside a grouped use statement requires an RFC, per se, but it will need to be explicitly added. @rust-lang/compiler?

[cramertj]

+1 for allowing attributes on use-statement subitems/groups.

[estebank]

I'm intrigued at what docs on imports would look like in the rendered docs. I can see the point for reexports (pub use), but not for the general case (other than in the code itself, at which point they are just comments).