Rollup merge of #23296 - shepmaster:explain-why-joinguard-is-must-use, r=alexcrichton

Author: Manishearth

src/libstd/thread.rs

@@ -635,7 +635,7 @@ impl Drop for JoinHandle {
 /// Due to platform restrictions, it is not possible to `Clone` this
 /// handle: the ability to join a child thread is a uniquely-owned
 /// permission.
-#[must_use]
+#[must_use = "thread will be immediately joined if `JoinGuard` is not used"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct JoinGuard<'a, T: 'a> {
     inner: JoinInner<T>,