`tbb::task_arena::enqueue` should at least document that execution order isn't guaranteed

[BenFrantzDale]

I was surprised to find that enqueueing into a tbb::task_arena{1} doesn't execute in FIFO order: https://godbolt.org/z/qz1hq3zaE (Although I realize now that for tbb::task_arena{1} I have arena.max_concurrency() == 2, which is not what I'd expect...)

At very least, the docs for tbb::task_arena::enqueue should be clear about this.

At best, it should be that there's a way to make a tbb::task_arena that I can enqueue FIFO work into and have order guaranteed for the case of max_concurrency == 1.