Indicate how the JoinHandle struct is created.

Author: frewsxcv

src/libstd/thread/mod.rs

@@ -610,6 +610,12 @@ impl<T> JoinInner<T> {
 /// Due to platform restrictions, it is not possible to `Clone` this
 /// handle: the ability to join a child thread is a uniquely-owned
 /// permission.
+///
+/// This `struct` is created by the [`thread::spawn`] function and the
+/// [`thread::Builder::spawn`] method.
+///
+/// [`thread::spawn`]: fn.spawn.html
+/// [`thread::Builder::spawn`]: struct.Builder.html#method.spawn
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct JoinHandle<T>(JoinInner<T>);