DOC: Extend docstring pandas core index to_frame method
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR provides adaptations of the original `to_frame` docstring, taking into account the [docstring guide](https://python-sprints.github.io/pandas/guide/pandas_docstring.html). 

Hello @stijnvanhoey! Thanks for updating the PR.

 - In the file [`pandas/core/indexes/base.py`](https://github.com/pandas-dev/pandas/blob/b32b4165e4c988ea67370ed1e03cdd10e758c6e2/pandas/core/indexes/base.py), following are the PEP8 issues :

> [Line 1133:15](https://github.com/pandas-dev/pandas/blob/b32b4165e4c988ea67370ed1e03cdd10e758c6e2/pandas/core/indexes/base.py#L1133): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace



##### Comment last updated on March 07, 2018 at 21:11 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20036?src=pr&el=h1) Report
> Merging [#20036](https://codecov.io/gh/pandas-dev/pandas/pull/20036?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fd010de51bd21e4240daac2e1fd0044c65a45091?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20036/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20036?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20036      +/-   ##
==========================================
+ Coverage   91.71%   91.71%   +<.01%     
==========================================
  Files         150      150              
  Lines       49104    49112       +8     
==========================================
+ Hits        45035    45045      +10     
+ Misses       4069     4067       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20036?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20036/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <ø> (ø)` | :arrow_up: |
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/20036/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.2% <0%> (+0.88%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20036?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/20036?src=pr&el=footer). Last update [fd010de...b32b416](https://codecov.io/gh/pandas-dev/pandas/pull/20036?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 There is indeed a difference between the online version and the `rst` source of the docstring guide. I used the online guide to do the adaptations. Fixed now