Unpin being an auto trait is not documented in list of auto traits

As documented in the [`Unpin`](https://doc.rust-lang.org/std/marker/trait.Unpin.html) stdlib docs, `Unpin` "is automatically implemented for almost every type." However, [the "Auto Traits" section](https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits) does not list it.