mention that extern absolute paths should gate on rust 2018

Author: Manishearth

src/librustc_resolve/lib.rs

@@ -3258,6 +3258,8 @@ impl<'a> Resolver<'a> {
                     let prev_name = path[0].name;
                     if prev_name == keywords::Extern.name() ||
                        prev_name == keywords::CrateRoot.name() &&
+                       // Note: When this feature stabilizes, this should
+                       // be gated on sess.rust_2018()
                        self.session.features_untracked().extern_absolute_paths {
                         // `::extern_crate::a::b`
                         let crate_id = self.crate_loader.process_path_extern(name, ident.span);