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>
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=h1) Report
> Merging [#20385](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7273ea0709590e6264607f227bb8def0ef656c50?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20385/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20385      +/-   ##
==========================================
+ Coverage   91.77%    91.8%   +0.02%     
==========================================
  Files         152      152              
  Lines       49205    49215      +10     
==========================================
+ Hits        45159    45181      +22     
+ Misses       4046     4034      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.84% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.29% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.84% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <100%> (ø)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.26% <0%> (-0.01%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.27% <0%> (ø)` | :arrow_up: |
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.93% <0%> (+0.06%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.11% <0%> (+0.16%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/20385/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=footer). Last update [7273ea0...c9a8a34](https://codecov.io/gh/pandas-dev/pandas/pull/20385?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
