Implemented #[doc(cfg(...))].

This attribute has two effects:

1. Items with this attribute and their children will have the "This is
   supported on **** only" message attached in the documentation.

2. The items' doc tests will be skipped if the configuration does not
   match.

src/test/compile-fail/feature-gate-doc_cfg.rs

@@ -0,0 +1,12 @@
+// 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.
+
+#[doc(cfg(unix))] //~ ERROR: #[doc(cfg(...))] is experimental
+fn main() {}