DOC: Clarify that DataFrame.sort_values is stable for sorting by multiple columns or labels
- [x] closes #38357
- ~~tests added / passed~~
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- ~~whatsnew entry~~

From what I understand from the code, for `by` containing more than one entry, `pandas.DataFrame.sort_values` uses `pandas.core.sorting.lexsort_indexer`, which converts the columns to ordered `Categorical`s, gets grouped lexicographic group indices via `get_group_index`, compressed these via `compress_group_index` and then sorts using counting sort or mergesort via `get_group_index_sorter`. Both of these are stable, as explained in `get_group_index_sorter`'s docstring. Therefore, the sorting should be stable if `sort_values` is called with `len(by)>1`.