Better description and warnings for using `n_workers=auto`

When we start dask-workers from the CLI, `--nworkers=auto` overrides any `--nthreads` specified, as mentioned here: https://github.com/dask/distributed/blob/a40f20553a9f6edd4e7072239744d71c6e0c0bcb/distributed/cli/dask_worker.py#L148

We can make this behavior clearer by:

1. Updating the [--nthreads description](https://github.com/dask/distributed/blob/a40f20553a9f6edd4e7072239744d71c6e0c0bcb/distributed/cli/dask_worker.py#L130) to mention that it is ignored if `--nworkers=auto`

2. Sharing a warning if both `nworkers=auto` and `nthreads` are specified by the user

**Minimal Complete Verifiable Example**:

```python
$ dask-scheduler

$ dask-worker tcp://192.168.1.48:8786 --nworkers auto --nthreads 1

>>> client=Client("tcp://192.168.1.48:8786") # see the workers tab in the dashboard
```