Into item level docs (enum Ordering)

Author: DirkyJerky

src/libcore/sync/atomic.rs

@@ -142,6 +142,8 @@ unsafe impl<T> Sync for AtomicPtr<T> {}
 ///
 /// Rust's memory orderings are [the same as
 /// LLVM's](http://llvm.org/docs/LangRef.html#memory-model-for-concurrent-operations).
+///
+/// For more information see the [nomicon](https://doc.rust-lang.org/nomicon/atomics.html).
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Copy, Clone, Debug)]
 pub enum Ordering {