Remove outdated descriptions from docs (#36)

Author: taiki-e

async-stream/README.md

@@ -138,9 +138,7 @@ fn bind_and_accept(addr: SocketAddr)
 ## Implementation
 
 The `stream!` and `try_stream!` macros are implemented using proc macros.
-Given that proc macros in expression position are not supported on stable
-rust, a hack similar to the one provided by the [`proc-macro-hack`] crate is
-used. The macro searches the syntax tree for instances of `sender.send($expr)` and
+The macro searches the syntax tree for instances of `sender.send($expr)` and
 transforms them into `sender.send($expr).await`.
 
 The stream uses a lightweight sender to send values from the stream

async-stream/src/lib.rs

@@ -145,9 +145,7 @@
 //! # Implementation
 //!
 //! The `stream!` and `try_stream!` macros are implemented using proc macros.
-//! Given that proc macros in expression position are not supported on stable
-//! rust, a hack similar to the one provided by the [`proc-macro-hack`] crate is
-//! used. The macro searches the syntax tree for instances of `sender.send($expr)` and
+//! The macro searches the syntax tree for instances of `sender.send($expr)` and
 //! transforms them into `sender.send($expr).await`.
 //!
 //! The stream uses a lightweight sender to send values from the stream