Rename JoinHandle::is_running to is_finished and update docs.

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

library/std/src/thread/mod.rs

@@ -1443,13 +1443,18 @@ impl<T> JoinHandle<T> {
         self.0.join()
     }
 
-    /// Checks if the associated thread is still running its main function.
+    /// Checks if the associated thread has finished running its main function.
     ///
-    /// This might return `false` for a brief moment after the thread's main
+    /// 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].
     #[unstable(feature = "thread_is_running", issue = "90470")]
-    pub fn is_running(&self) -> bool {
-        Arc::strong_count(&self.0.packet) > 1
+    pub fn is_finished(&self) -> bool {
+        Arc::strong_count(&self.0.packet) == 1
     }
 }