Rollup merge of #46858 - QuietMisdreavus:external-doc-error, r=estebank

tweaks and fixes for doc(include)

This PR makes a handful of changes around `#[doc(include="file.md")]` (https://github.com/rust-lang/rust/issues/44732):

* Turns errors when loading files into full errors. This matches the original RFC text.
* Makes the `missing_docs` lint check for `#[doc(include="file.md")]` as well as regular `#[doc="text"]` attributes.
* Loads files included by `#[doc(include="file.md")]` into dep-info, mirroring the behavior of `include_str!()` and friends.
* Adds or modifies tests to check for all of these.

src/test/compile-fail/external-doc-error.rs

@@ -0,0 +1,16 @@
+// Copyright 2012-2015 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)]
+
+#[doc(include = "not-a-file.md")] //~ ERROR: couldn't read
+pub struct SomeStruct;
+
+fn main() {}