Add doc for fn keyword

Author: GuillaumeGomez

src/libstd/keyword_docs.rs

@@ -0,0 +1,28 @@
+// 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.
+
+#[doc(keyword = "fn")]
+//
+/// The `fn` keyword.
+///
+/// The `fn` keyword is used to declare a function.
+///
+/// Example:
+///
+/// ```rust
+/// fn some_function() {
+///     // code goes in here
+/// }
+/// ```
+///
+/// For more information about functions, take a look at the [Rust Book][book].
+///
+/// [book]: https://doc.rust-lang.org/book/second-edition/ch03-03-how-functions-work.html
+mod fn_keyword { }

src/libstd/lib.rs

@@ -547,3 +547,8 @@ pub use stdsimd::arch;
 // the rustdoc documentation for primitive types. Using `include!`
 // because rustdoc only looks for these modules at the crate level.
 include!("primitive_docs.rs");
+
+// Include a number of private modules that exist solely to provide
+// the rustdoc documentation for the existing keywords. Using `include!`
+// because rustdoc only looks for these modules at the crate level.
+include!("keyword_docs.rs");