Re-exported extern_types are not rendered by rustdoc

[crumblingstatue]

#![feature(extern_types)]

extern {
    /// A nice extern type.
    pub type Foo;
}

mod sub {
    extern {
        /// Another extern type. It's not available in the rendered documentaiton.
        pub type Bar;
    }
    /// A non-extern type. This is rendered correctly.
    pub struct NonExtern;
}

pub use sub::{Bar, NonExtern};

Rendered:

EDIT: Updated example with non-extern type for contrast.

[crumblingstatue]

cc @kennytm