Add keyword docs on enum

Author: iirelu

src/libstd/keyword_docs.rs

@@ -124,6 +124,61 @@ mod const_keyword { }
 /// [`pub`]: keyword.pub.html
 mod crate_keyword { }
 
+#[doc(keyword = "enum")]
+//
+/// For defining enumerations.
+///
+/// Enums in Rust are similar to those of other compiled languages like C, but have important
+/// differences that make them considerably more powerful. What Rust calls enums are more commonly
+/// known as Algebraic Data Types if you're coming from a functional programming background, but
+/// the important part is that data can go with the enum variants.
+///
+/// ```rust
+/// # struct Coord;
+/// enum SimpleEnum {
+///     FirstVariant,
+///     SecondVariant,
+///     ThirdVariant,
+/// }
+///
+/// enum Location {
+///     Unknown,
+///     Anonymous,
+///     Known(Coord),
+/// }
+///
+/// enum ComplexEnum {
+///     Nothing,
+///     Something(u32),
+///     LotsOfThings {
+///         usual_struct_stuff: bool,
+///         blah: String,
+///     }
+/// }
+///
+/// enum EmptyEnum { }
+/// ```
+///
+/// The first enum shown is the usual kind of enum you'd find in a C-style language. The second
+/// shows off a hypothetical example of something storing location data, with Coord being any other
+/// type that's needed, for example a struct. The third example demonstrates the kind of variant a
+/// variant can store, ranging from nothing, to a tuple, to an anonymous struct.
+///
+/// Instantiating enum variants involves explicitly using the enum's name as its namespace,
+/// followed by one of its variants. `SimpleEnum::SecondVariant` would be an example from above.
+/// When data follows along with a variant, such as with rust's built-in [`Option`] type, the data
+/// is added as the type describes, for example `Option::Some(123)`. The same follows with
+/// struct-like variants, with things looking like `ComplexEnum::LotsOfThings { usual_struct_stuff:
+/// true, blah: "hello!".to_string(), }`. Empty Enums are similar to () in that they cannot be
+/// instantiated at all, and are used mainly to mess with the type system in interesting ways.
+///
+/// For more information, take a look at the [Rust Book] or the [Reference]
+///
+/// [`Option`]: option/enum.Option.html
+/// [Rust Book]: https://doc.rust-lang.org/book/second-edition/ch06-01-defining-an-enum.html
+/// [Reference]: https://doc.rust-lang.org/reference/items/enumerations.html
+mod enum_keyword { }
+
 #[doc(keyword = "fn")]
 //
 /// The `fn` keyword.