allow loading external files in documentation

Partial implementation of https://github.com/rust-lang/rfcs/pull/1990
(needs error reporting work)

cc #44732

src/test/rustdoc/external-doc.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(external_doc)]
+
+// @has external_doc/struct.CanHasDocs.html
+// @has - '//h1' 'External Docs'
+// @has - '//h2' 'Inline Docs'
+#[doc(include = "auxiliary/external-doc.md")]
+/// ## Inline Docs
+pub struct CanHasDocs;