Flaky doc test in bevy_tasks

[hymm]

Bevy version

Test was added in #4466

What you did

Reported by targrub in discord https://discord.com/channels/691052431525675048/692572690833473578/1032353522475348018.

Test here (https://github.com/bevyengine/bevy/blob/main/crates/bevy_tasks/src/task_pool.rs#L148-L173) fails in about 1 in 20 runs of cargo run -p bevy_tasks

What went wrong

I wrote the doc test thinking that it was impossible for the task that spawns another task to be inserted into the scope's task queue after the task that it spawns. That was an incorrect assumption.

    pub fn spawn<Fut: Future<Output = T> + 'scope + Send>(&self, f: Fut) {
        let task = self.executor.spawn(f);
        // ConcurrentQueue only errors when closed or full, but we never
        // close and use an unbouded queue, so it is safe to unwrap
        self.spawned.push(task).unwrap();
    }

The second task can be spawned between spawning the first task on the executor and inserting the tasks into the spawned queue.

This isn't really a problem as the docs state that a task spawned inside another task has non deterministic ordering. The fix should probably to just remove the assert and make it more clear when the results are deterministic and when it isn't.

[hymm]

I'll try and put together a PR for this soon.