DOC: enable docstring on DataFrame.columns/index
Follow-up on https://github.com/pandas-dev/pandas/pull/18196, similar to https://github.com/pandas-dev/pandas/pull/19991

Didn't add an actual full docstring yet, but this already gives:

```
In [1]: pd.DataFrame.columns?
Type:        AxisProperty
String form: <pandas._libs.properties.AxisProperty object at 0x7fa975d74da0>
File:        ~/scipy/pandas/pandas/_libs/properties.cpython-35m-x86_64-linux-gnu.so
Docstring:   columns
```

instead of (on master, on 0.22 this raises an exception):

```
In [1]: pd.DataFrame.columns?
Type:        NoneType
String form: None
Docstring:   <no docstring>
```