Rustdoc: exposes private modules that reexport functions

[mstewartgallus]

The following code makes rustdoc show the module to the outside world when the module should be private.

#[crate_type = "lib"];

mod should_not_be_visible_outside_of_lib {
    pub use self::context::function_exposing_bug;

    mod context {
        pub fn function_exposing_bug() {}
    }
}

[alexcrichton]

Closed by #11391