Document Self::Variant for enums

[adamchalmers]

In Rust 1.37 you can use type aliases for enums. I think it's worth showing people how that works in the page on Enums.

In my code, I only ever use this feature when I want to refer to an enum by Self instead of by its name (usually if the name is very long). I'm planning to use that as my example and link to the release notes/RFC if readers want more info.