DOC: Clarify stability of sorting in documentation of DataFrame.sort_values for multiple columns / labels
#### Location of the documentation

Description of the argument `kind` in the documentation of [pandas.DataFrame.sort_values](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.sort_values.html).

#### Documentation problem

The current documentation states (emphasize mine):

>Choice of sorting algorithm. See also ndarray.np.sort for more information. **mergesort is the only stable algorithm. For DataFrames, this option is only applied when sorting on a single column or label**.

To me that means, that the `kind` argument is ignored if you sort by multiple columns / labels. But I think one could even interpret it as `mergesort` explicitely is not done in that case. Running some tests, it does look like the sorting is stable when sorting multiple columns / labels. But it is not written anywhere. If it really is guaranteed that the sorting is stable for sorting by multiple columns / labels, it would be nice to have this written down explicitely in the documentation.

#### Suggested fix for documentation

IF that is actually the case, I would suggest to add a sentence to the effect that the sorting is stable if sorting is done by multiple columns / labels. E.g. change the `kind` argument description to:

>Choice of sorting algorithm. See also ndarray.np.sort for more information. mergesort is the only stable algorithm. ~~For DataFrames, this option is only applied when sorting on a single column or label.~~ **For DataFrames, if sorting by multiple columns or labels, this argument is ignored, defaulting to a stable sorting algorithm.**