fix failing doc test and clear up docs (#6314)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/6306

## Solution
 Change the failing assert and expand example to explain when ordering is deterministic or not.

Co-authored-by: Mike Hsu <mike.hsu@gmail.com>

crates/bevy_tasks/src/task_pool.rs

@@ -164,11 +164,19 @@ impl TaskPool {
     ///     });
     /// });
     ///
-    /// // results are returned in the order the tasks are spawned in.
+    /// // The ordering of results is non-deterministic if you spawn from within tasks as above.
-    /// // Note: the ordering may become non-deterministic if you spawn from within tasks.
+    /// // If you're doing this, you'll have to write your code to not depend on the ordering.
-    /// // the ordering is only guaranteed when tasks are spawned directly from the main closure.
+    /// assert!(results.contains(&0));
+    /// assert!(results.contains(&1));
+    ///
+    /// // The ordering is deterministic if you only spawn directly from the closure function.
+    /// let results = pool.scope(|s| {
+    ///     s.spawn(async { 0  });
+    ///     s.spawn(async { 1 });
+    /// });
     /// assert_eq!(&results[..], &[0, 1]);
-    /// // can access x after scope runs
+    ///
+    /// // You can access x after scope runs, since it was only temporarily borrowed in the scope.
     /// assert_eq!(x, 2);
     /// ```
     ///