docs: Enable required feature for 'closure_returning_async_block' lint

Failing to do this results in the lint example output complaining
about the lint not existing instead of the thing the lint is supposed
to be complaining about.

Author: Fayti1703

Diff for: compiler/rustc_lint/src/async_closures.rs

@@ -12,6 +12,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
+    /// #![feature(async_closure)]
     /// #![warn(closure_returning_async_block)]
     /// let c = |x: &str| async {};
     /// ```