note that async-await is only available in rust2018

Author: nikomatsakis

src/expressions/await-expr.md

@@ -29,6 +29,8 @@ More specifically, an `<expr>.await` expression has the following effect.
 [`Poll::Pending`]: https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Pending
 [`Poll::Ready`]: https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Ready
 
+> **Edition differences**: Await expressions are only available in Rust 2018.
+
 ## Task context
 
 The task context refers to the [`Context`] which was supplied to the

src/expressions/block-expr.md

@@ -99,6 +99,8 @@ this type is unspecified, but it is roughly equivalent 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.
+
 ### Capture modes
 
 Async blocks capture variables from their environment using the same
@@ -133,7 +135,7 @@ of the future.
 Finally, the `break` and `continue` keywords cannot be used to branch
 out from an async block. Therefore the following is illegal:
 
-```rust,compile_fail
+```rust,edition2018,compile_fail
 loop {
     async move {
         break; // This would break out of the loop.

src/items/functions.md

@@ -236,6 +236,8 @@ For more information on the effect of async, see [`async` blocks][`async` block]
 [`async move` block]: ../expressions/block-expr.html#async-blocks
 [`impl Future`]: ../types/impl-trait.html
 
+> **Edition differences**: Async functions are only available in Rust 2018.
+
 ## Attributes on functions
 
 [Outer attributes][attributes] are allowed on functions. [Inner