DOC: .nlargest/smallest docs specify keep=False as valid parameter
For both DataFrame and Series docs:

> keep : {‘first’, ‘last’, False}, default ‘first’
Where there are duplicate values: - first : take the first occurrence. - last : take the last occurrence.

```
In [2]: pd.Series(range(5)).nlargest(3, keep=False)

ValueError: keep must be either "first", "last"

In [3]: pd.DataFrame(range(5)).nsmallest(3,  0, keep=False)

ValueError: keep must be either "first", "last"
```