LocalCluster and dask-worker CLI default behaviour for n_workers and threads_per_worker isn't documented clearly.

Came across [this SO question](https://stackoverflow.com/questions/71494237/dask-how-the-memory-limit-is-calculated-in-auto-mode/71498481#71498481) and I had previously also spent some time trying to figure out the default behaviour for how `n_workers` and `threads_per_worker` are being calculated when using `LocalCluster` or the `dask-worker` CLI.

It seems there are sane defaults ([source](https://github.com/dask/distributed/blob/main/distributed/deploy/local.py#L185-L197)) but the [docs](http://distributed.dask.org/en/stable/api.html#distributed.LocalCluster) don't describe those.

I'd like to create a docs PR for this, but I may be missing something here?