Editions "in" -> "beginning with".

In case there is an edition after 2018, so that this is clear what it covers.

src/expressions/await-expr.md

@@ -33,7 +33,8 @@ More specifically, an `<expr>.await` expression has the following effect.
 [`Poll::Pending`]: ../../std/task/enum.Poll.html#variant.Pending
 [`Poll::Ready`]: ../../std/task/enum.Poll.html#variant.Ready
 
-> **Edition differences**: Await expressions are only available in Rust 2018.
+> **Edition differences**: Await expressions are only available beginning with
+> Rust 2018.
 
 ## Task context
 

src/expressions/block-expr.md

@@ -101,7 +101,7 @@ this type is unspecified.
 > to an enum with one variant per `await` point, where each variant
 > stores the data needed to resume from its corresponding point.
 
-> **Edition differences**: Async blocks are only available in Rust 2018.
+> **Edition differences**: Async blocks are only available beginning with Rust 2018.
 
 [`std::ops::Fn`]: ../../std/ops/trait.Fn.html
 [`std::future::Future`]: ../../std/future/trait.Future.html

src/items/functions.md

@@ -240,7 +240,8 @@ For more information on the effect of async, see [`async` blocks][async-blocks].
 [async-blocks]: ../expressions/block-expr.md#async-blocks
 [`impl Future`]: ../types/impl-trait.md
 
-> **Edition differences**: Async functions are only available in Rust 2018.
+> **Edition differences**: Async functions are only available beginning with
+> Rust 2018.
 
 ## Attributes on functions