Add note linking to Rust 2018 path semantics docs.

This commit extends existing path suggestions to link to documentation
on the changed semantics of `use` in Rust 2018.

src/test/ui/rust-2018/local-path-suggestions-2018.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `foo`
    |
 LL |     use foo::Bar;
    |         ^^^ Did you mean `crate::foo`?
+   |
+   = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>
 
 error[E0432]: unresolved import `foo`
   --> $DIR/local-path-suggestions-2018.rs:27:5