#[doc(hidden)] does not work for externally used macros

[SergioBenitez]

If a macro is imported using use (via the use_extern_macros feature), its documentation cannot be hidden. For instance, if the code is:

extern crate serde_json;

#[doc(hidden)]
pub use serde_json::json;

The documentation for the json macro will appear in the rustdocs. Of course, the expected result is that the documentation does not appear.

cc @jseyfried @nrc @dtolnay

[abonander]

I'm going to tinker on this while I'm out of town over the weekend. @jseyfried bug me about this Monday evening-ish if I don't have a progress report by then.