Auto merge of #29529 - Ryman:rustdoc-cap-lints, r=alexcrichton

This sets the `cap-lints` setting to 'allow' for all doc compilations. There's precedent for this as rustdoc [already whitelists unstable code](https://github.com/rust-lang/rust/blob/master/src/librustdoc/core.rs#L112) when compiling documentation, with the expectation being that a regular compile will complain about any problems. I think the same justification applies here.

Problem case in the wild: https://github.com/laumann/compiletest-rs/pull/28

r? @Manishearth

src/test/rustdoc/cap-lints.rs

@@ -0,0 +1,20 @@
+// Copyright 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.
+
+// This should fail a normal compile due to non_camel_case_types,
+// It should pass a doc-compile as it only needs to type-check and
+// therefore should not concern itself with the lints.
+#[deny(warnings)]
+
+// @has cap_lints/struct.foo.html //pre '#[must_use]'
+#[must_use]
+pub struct foo {
+    field: i32,
+}