MultiLevelIndex sort_index already does support ascending as a list of booleans
#### Code Sample, a copy-pastable example if possible

```python
df.sort_index(level=[1, 0], ascending=[True, False])
```
#### Problem description

The [documentation](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.sort_index.html) states that `pandas.DataFrame.sort_index` parameter `accepts` is a boolean. However, in the case of multi level index, it can (already) be a list of booleans indicating which level should ascending and which descending.

Both [lexsort_indexer](https://github.com/pandas-dev/pandas/blob/master/pandas/core/sorting.py#L192) and [sort_levels](https://github.com/pandas-dev/pandas/blob/4181042aad88224c9c8be8e7c7a8525090c8aa07/pandas/core/frame.py#L5031) support this behavior.

I suggest updating both tests and documentation.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.14-arch1-1
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF8
LOCALE           : en_US.UTF-8

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0
Cython           : 0.29.14
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.11.1
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
