Add let keyword doc

src/libstd/keyword_docs.rs

@@ -1,4 +1,4 @@
-// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -26,3 +26,33 @@
 ///
 /// [book]: https://doc.rust-lang.org/book/second-edition/ch03-03-how-functions-work.html
 mod fn_keyword { }
+
+#[doc(keyword = "let")]
+//
+/// The `let` keyword.
+///
+/// The `let` keyword is used to declare a variable.
+///
+/// Example:
+///
+/// ```rust
+/// # #![allow(unused_assignments)]
+/// let x = 3; // We create a variable named `x` with the value `3`.
+/// ```
+///
+/// By default, all variables are **not** mutable. If you want a mutable variable,
+/// you'll have to use the `mut` keyword.
+///
+/// Example:
+///
+/// ```rust
+/// # #![allow(unused_assignments)]
+/// let mut x = 3; // We create a mutable variable named `x` with the value `3`.
+///
+/// x += 4; // `x` is now equal to `7`.
+/// ```
+///
+/// For more information about the `let` keyword, take a look at the [Rust Book][book].
+///
+/// [book]: https://doc.rust-lang.org/book/second-edition/ch03-01-variables-and-mutability.html
+mod let_keyword { }