Document ordering bwetween future drop and JoinHandle completion

Author: stepancheg

tokio/src/runtime/task/join.rs

@@ -22,6 +22,11 @@ cfg_rt! {
     /// This `struct` is created by the [`task::spawn`] and [`task::spawn_blocking`]
     /// functions.
     ///
+    /// It is guaranteed that the spawned task's destructor has finished
+    /// before the task completion is observed
+    /// via [`JoinHandle`] `await`, [`JoinHandle::is_finished`] or
+    /// [`AbortHandle::is_finished](crate::task::AbortHandle::is_finished).
+    ///
     /// # Cancel safety
     ///
     /// The `&mut JoinHandle<T>` type is cancel safe. If it is used as the event