Add test for documenting the re-export

Author: jyn514

src/test/rustdoc/intra-doc-crate/additional_doc.rs

@@ -0,0 +1,8 @@
+// aux-build:additional_doc.rs
+// build-aux-docs
+extern crate rand;
+
+// @has 'additional_doc/trait.Rng.html' '//a[@href="../additional_doc/trait.Rng.html"]' 'Rng'
+// @has 'additional_doc/trait.Rng.html' '//a[@href="../rand/trait.RngCore.html"]' 'RngCore'
+/// This is an [`Rng`].
+pub use rand::Rng;

src/test/rustdoc/intra-doc-crate/auxiliary/additional_doc.rs

@@ -0,0 +1,5 @@
+#![crate_name = "rand"]
+
+pub trait RngCore {}
+/// Rng extends [`RngCore`].
+pub trait Rng: RngCore {}