Better example for thread::unpark.

Part of #29378

Author: Felix Raimundo

src/libstd/thread/mod.rs

@@ -769,7 +769,7 @@ impl Thread {
     /// Atomically makes the handle's token available if it is not already.
     ///
     /// Every thread is equipped with some basic low-level blocking support, via
-    /// the [`park()`][park] function and the `unpark` method. These can be
+    /// the [`park()`][park] function and the `unpark()` method. These can be
     /// used as a more CPU-efficient implementation of a spinlock.
     ///
     /// See the [module doc][thread] for more detail.
@@ -779,14 +779,21 @@ impl Thread {
     /// ```
     /// use std::thread;
     ///
-    /// let handler = thread::Builder::new()
+    /// let parked_thread = thread::Builder::new()
     ///     .spawn(|| {
-    ///         let thread = thread::current();
-    ///         thread.unpark();
+    ///         println!("Parking thread");
+    ///         thread::park();
+    ///         println!("Thread unparked");
     ///     })
     ///     .unwrap();
     ///
-    /// handler.join().unwrap();
+    /// // Let some time pass for the thread to be spawned.
+    /// thread::sleep(Duration::from_millis(10));
+    ///
+    /// println!("Unpark the thread");
+    /// parked_thread.thread().unpark();
+    ///
+    /// parked_thread.join().unwrap();
     /// ```
     ///
     /// [thread]: index.html