Document the mod keyword

Author: poliorcetics

src/libstd/keyword_docs.rs

@@ -913,10 +913,28 @@ mod match_keyword {}
 //
 /// Organize code into [modules].
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// Use `mod` to create new [modules] to encapsulate code, including other
+/// modules:
 ///
+/// ```
+/// mod foo {
+///     mod bar {
+///         type MyType = (u8, u8);
+///         fn baz() {}
+///     }
+/// }
+/// ```
+///
+/// Like [`struct`]s and [`enum`]s, a module and its content are private by
+/// default, unaccessible to code outside of the module.
+///
+/// To learn more about allowing access, see the documentation for the [`pub`]
+/// keyword.
+///
+/// [`enum`]: keyword.enum.html
+/// [`pub`]: keyword.pub.html
+/// [`struct`]: keyword.struct.html
 /// [modules]: ../reference/items/modules.html
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
 mod mod_keyword {}
 
 #[doc(keyword = "move")]