API/DOC: types covered by lib.infer_dtype
In #23167, I'm trying consistently infer the dtype of the underlying Series/Index while calling the constructor of the `.str`-accessor. For testing this thoroughly, I wanted to build a parametrized fixture that returns an ndarray for all the dtypes that `lib.infer_dtype` can infer. I based myself on the list in the docstring, but found the following:
* the docstring mentions `'complex'` as a possible outcome, but this does not work (instead returning `'mixed'`)
```
>>> lib.infer_dtype([1+1j, 2+2j])
'mixed'
>>> lib.infer_dtype([np.complex128(1+1j)])
'mixed'
```
and I don't believe it's actually possible to achieve this, given the code.
* the docstring mentions `'timedelta64'`, but this similarly does not work (returning `'timedelta'`; and can't be hit either, IMO)
```
>>> lib.infer_dtype([np.timedelta64(1, 'D')])
'timedelta'
```
* the docstring *does not* mention `'interval'`, but that *is* a possible outcome:
```
>>> lib.infer_dtype([pd.Interval(0, 1), pd.Interval(0, 2)])
'interval'
```

So it needs to be discussed if `'complex'`/`'timedelta64'` should be added to the code or removed from the docstring, and vice versa for `'interval'`.