DOC: Basic indexing on axis with Multiindex 
#### Location of the documentation

[Basic indexing on axis with MultiIndex](https://pandas.pydata.org/pandas-docs/stable/user_guide/advanced.html#basic-indexing-on-axis-with-multiindex)


#### Documentation problem

The following example seems not to work:

```python
arrays = [
         np.array(["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"]),
         np.array(["one", "two", "one", "two", "one", "two", "one", "two"]),
     ]
    
df = pd.DataFrame(np.random.randn(8, 4), index=arrays)

df["bar"]
```

The code throws an error:

```python
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-13-6ea2f6413a49> in <module>
----> 1 df["bar"]

~/anaconda3/envs/jupyter/lib/python3.8/site-packages/pandas/core/frame.py in __getitem__(self, key)
   3022             if self.columns.nlevels > 1:
   3023                 return self._getitem_multilevel(key)
-> 3024             indexer = self.columns.get_loc(key)
   3025             if is_integer(indexer):
   3026                 indexer = [indexer]

~/anaconda3/envs/jupyter/lib/python3.8/site-packages/pandas/core/indexes/range.py in get_loc(self, key, method, tolerance)
    352                 except ValueError as err:
    353                     raise KeyError(key) from err
--> 354             raise KeyError(key)
    355         return super().get_loc(key, method=method, tolerance=tolerance)
    356 

KeyError: 'bar'
```

When asking for help, it is suggested to use `df.loc["bar"]` instead with as shown in another part of the documentation ([advanced indexing with hierarchical index](https://pandas.pydata.org/docs/dev/user_guide/advanced.html#advanced-indexing-with-hierarchical-index)).

#### Suggested fix for documentation

An explanation for when partial indexing without the usage of `.loc[]` is applicable would be helpful or choosing an example that works in the current version. Trying to reproduce documentation code that does not work leads to confusion.
