DOC: fix doc build warnings
Bunch of small fixes that caused warnings/errors
Yes, I know (and think there is an issue about that somewhere). The problem is that the current doc build is not yet fully warning-free (also not after this PR), of which some I don't directly know how to fix. So as long that is not the case, we cannot let it fail on that. We could let sphinx fail on a a warning/error, or otherwise parse the log and if certain words are present (like 'ERROR') then let travis fail (and in this way, be a bit more flexible on what is not allowed to fail).

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15505?src=pr&el=h1) Report
> Merging [#15505](https://codecov.io/gh/pandas-dev/pandas/pull/15505?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc?src=pr&el=desc) will **decrease** coverage by `-0.01%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15505      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         136      136              
  Lines       49553    49553              
==========================================
- Hits        44781    44780       -1     
- Misses       4772     4773       +1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15505?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/html.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?src=pr&el=tree#diff-cGFuZGFzL2lvL2h0bWwucHk=) | `84.48% <ø> (ø)` | :white_check_mark: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.51% <ø> (ø)` | :white_check_mark: |
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `97.87% <ø> (ø)` | :white_check_mark: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.31% <ø> (ø)` | :white_check_mark: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <ø> (-0.34%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15505?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/15505?src=pr&el=footer). Last update [d80275d...172a85e](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...172a85ef0503198f700c2c09b9460d6103efe64b?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).