DOC: Unused `order` argument in factorize
``` python
def factorize(values, sort=False, order=None, na_sentinel=-1):
    """
    Encode input values as an enumerated type or categorical variable

    Parameters
    ----------
    values : ndarray (1-d)
        Sequence
    sort : boolean, default False
        Sort by values
    order :
    na_sentinel: int, default -1
        Value to mark "not found"

    Returns
    -------
    labels : the indexer to the original array
    uniques : the unique values

    note: an array of Periods will ignore sort as it returns an always sorted PeriodIndex
    """
```

But it isn't used anywhere. Technically this is an API change, if people aren't using kwargs. So I'll add it to the deprecation list.
