DOC: .get_slice_bound in MultiIndex needs documentation.
```
In [10]: i = Index(['c','a','d','b'])                                           
In [12]: i.get_slice_bound('a',side="left",kind="ix")                           
Out[12]: 1
In [13]: mi = MultiIndex.from_arrays([['c','a','d','b']])                       
In [15]: mi.get_slice_bound('a',side="left",kind="ix")                          
---------------------------------------------------------------------------
UnsortedIndexError                        Traceback (most recent call last)
<ipython-input-15-fbc28595630f> in <module>
----> 1 mi.get_slice_bound('a',side="left",kind="ix")
~/anaconda3/lib/python3.7/site-packages/pandas/core/indexes/multi.py in get_slice_bound(self, label, side, kind)
   2523         if not isinstance(label, tuple):
   2524             label = (label,)
-> 2525         return self._partial_tup_index(label, side=side)
   2526 
   2527     def slice_locs(self, start=None, end=None, step=None, kind=None):
~/anaconda3/lib/python3.7/site-packages/pandas/core/indexes/multi.py in _partial_tup_index(self, tup, side)
   2585             raise UnsortedIndexError(
   2586                 "Key length (%d) was greater than MultiIndex"
-> 2587                 " lexsort depth (%d)" % (len(tup), self.lexsort_depth)
   2588             )
   2589 
UnsortedIndexError: 'Key length (1) was greater than MultiIndex lexsort depth (0)'
```

But, If `MultiIndex` is sorted,

```
In [17]: mi = MultiIndex.from_arrays([['a','b','c','d']])                       
In [18]: mi.get_slice_bound('a',side="left",kind="ix")                          
Out[18]: 0
``` 

If only sorted 'MultiIndex' can work is not bug  but intended it. then I think need documentation for 'MultiIndex'.  Because just  documentation on 'Index.get_slice_bound' is not enough to recognize '.get_slice_bound' only works with sorted 'MultiIndex'



<details>
INSTALLED VERSIONS

------------------
commit           : None
python           : 3.7.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.0.0-36-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : ko_KR.UTF-8
LOCALE           : ko_KR.UTF-8

pandas           : 0.25.1
numpy            : 1.16.1
pytz             : 2018.7
dateutil         : 2.7.5
pip              : 19.3.1
setuptools       : 40.6.3
Cython           : 0.29.2
pytest           : 5.1.0
hypothesis       : None
sphinx           : 1.8.2
blosc            : None
feather          : None
xlsxwriter       : 1.1.2
lxml.etree       : 4.2.5
html5lib         : 1.0.1
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.2.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.5
matplotlib       : 3.0.2
numexpr          : 2.6.8
odfpy            : None
openpyxl         : 2.5.12
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : 1.2.15
tables           : 3.4.4
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.2


</details>
