Improve docs for is_running to explain use case

library/std/src/thread/mod.rs

@@ -1487,13 +1487,14 @@ impl<T> JoinHandle<T> {
 
     /// Checks if the associated thread has finished running its main function.
     ///
+    /// `is_finished` supports implementing a non-blocking join operation, by checking
+    /// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
+    /// block while waiting on the thread to finish, use [`join`][Self::join].
+    ///
     /// This might return `true` for a brief moment after the thread's main
     /// function has returned, but before the thread itself has stopped running.
     /// However, once this returns `true`, [`join`][Self::join] can be expected
     /// to return quickly, without blocking for any significant amount of time.
-    ///
-    /// This function does not block. To block while waiting on the thread to finish,
-    /// use [`join`][Self::join].
     #[stable(feature = "thread_is_running", since = "1.61.0")]
     pub fn is_finished(&self) -> bool {
         Arc::strong_count(&self.0.packet) == 1

library/std/src/thread/scoped.rs

@@ -308,13 +308,14 @@ impl<'scope, T> ScopedJoinHandle<'scope, T> {
 
     /// Checks if the associated thread has finished running its main function.
     ///
+    /// `is_finished` supports implementing a non-blocking join operation, by checking
+    /// `is_finished`, and calling `join` if it returns `false`. This function does not block. To
+    /// block while waiting on the thread to finish, use [`join`][Self::join].
+    ///
     /// This might return `true` for a brief moment after the thread's main
     /// function has returned, but before the thread itself has stopped running.
     /// However, once this returns `true`, [`join`][Self::join] can be expected
     /// to return quickly, without blocking for any significant amount of time.
-    ///
-    /// This function does not block. To block while waiting on the thread to finish,
-    /// use [`join`][Self::join].
     #[stable(feature = "scoped_threads", since = "1.63.0")]
     pub fn is_finished(&self) -> bool {
         Arc::strong_count(&self.0.packet) == 1