select! warning on recursion limit

Warning as discussion in issue rust-lang#1917.

Author: rubdos

futures-util/src/async_await/select_mod.rs

@@ -32,6 +32,9 @@ macro_rules! document_select_macro {
         /// It is also gated behind the `async-await` feature of this library, which is
         /// _not_ activated by default.
         ///
+        /// **Warning**: `select!` relies on `proc-macro-hack`, and may require to set the
+        /// compiler's recursion limit very high, e.g. `#![recursion_limit="1024"]`.
+        ///
         /// # Examples
         ///
         /// ```