task: add missing feature flags for task_local (#3236)

Author: nylonicious

tokio/src/task/task_local.rs

@@ -31,6 +31,7 @@ use std::{fmt, thread};
 ///
 /// [`tokio::task::LocalKey`]: struct@crate::task::LocalKey
 #[macro_export]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "rt-util", feature = "rt-core"))))]
 macro_rules! task_local {
      // empty (base case for the recursion)
     () => {};
@@ -90,6 +91,7 @@ macro_rules! __task_local_inner {
 /// # }
 /// ```
 /// [`std::thread::LocalKey`]: struct@std::thread::LocalKey
+#[cfg_attr(docsrs, doc(cfg(all(feature = "rt-util", feature = "rt-core"))))]
 pub struct LocalKey<T: 'static> {
     #[doc(hidden)]
     pub inner: thread::LocalKey<RefCell<Option<T>>>,