DOC: Fix warnings in doc build
Split from https://github.com/pandas-dev/pandas/pull/22743

This has just the docs changes, not the doc build / CI changes, so I don't have to keep fixing merge conflicts.
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/TomAugspurger/pandas/blob/2947688bd38a811d4f7fe2e1fc0bee31e62b752a/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/TomAugspurger/pandas/blob/2947688bd38a811d4f7fe2e1fc0bee31e62b752a/pandas/core/series.py) !

 - There are no PEP8 issues in the file [`pandas/core/window.py`](https://github.com/TomAugspurger/pandas/blob/2947688bd38a811d4f7fe2e1fc0bee31e62b752a/pandas/core/window.py) !

 - There are no PEP8 issues in the file [`pandas/io/formats/style.py`](https://github.com/TomAugspurger/pandas/blob/2947688bd38a811d4f7fe2e1fc0bee31e62b752a/pandas/io/formats/style.py) !

 Latest commit fixes a failing doctest on master

https://travis-ci.org/pandas-dev/pandas/jobs/433544423#L2688

```
2059             columns of two DataFrame objects.
2060 
2061         Notes
2062         -----
2063         If the Pearson correlation is not well defined return 'NaN'.
2064 
2065         Examples
2066         --------
2067         >>> s = pd.Series([0.25, 0.5, 0.2, -0.05])
2068         >>> s.autocorr()
Expected:
    0.1035526330902407
Got:
    0.10355263309024071
```

Assuming it's a floating point precision issue. @jorisvandenbossche @datapythonista are you OK with using `doctest: +ELLIPSIS` for this? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22838?src=pr&el=h1) Report
> Merging [#22838](https://codecov.io/gh/pandas-dev/pandas/pull/22838?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f64f9940265baeba3658fa5ad87f03d7d5ad25a8?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/22838/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22838?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22838      +/-   ##
==========================================
+ Coverage   92.19%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50825    50827       +2     
==========================================
+ Hits        46857    46860       +3     
+ Misses       3968     3967       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.37% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22838?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.43% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.74% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.67% <ø> (ø)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.4% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.2% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22838/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.66% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22838?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/22838?src=pr&el=footer). Last update [f64f994...c9b1921](https://codecov.io/gh/pandas-dev/pandas/pull/22838?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Merging in a bit, since master is failing. HTTP error downloading conda on that circle-ci one. The others passed, so merging. Thanks a lot for this!