Tracking issue for using impl Trait in more places

[carols10cents]

Rust tracking issue: rust-lang/rust#34511

Place that will need to be updated: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/trait-system/impl-trait-for-returning-complex-types-with-ease.md#L161-L168

Using impl Trait in more places

As previously mentioned, as a start, you will only be able to use impl Trait as the argument or return type of a free or inherent function. However, impl Trait can't be used inside implementations of traits, nor can it be used as the type of a let binding or inside a type alias. Some of these restrictions will eventually be lifted. For more information, see the tracking issue on impl Trait.

[ehuss]

Closing per #232, this section has been removed. Hopefully has new language changes are stabilized, other documentation will get updated.