DOC: Fix various warnings
I'm trying to get our doc build cleaned up. Just fixing warnings in this one
(mostly formatting, some references).

Later on I'll figure out why things are slow on sphinx 1.6 and get rid of our hacked numpydoc / ipython directive.
Last build on master had 52 warnings: https://s3.amazonaws.com/archive.travis-ci.org/jobs/296227387/log.txt?X-Amz-Expires=30&X-Amz-Date=20171102T220435Z&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAJRYRXRSVGNKPKO5A/20171102/us-east-1/s3/aws4_request&X-Amz-SignedHeaders=host&X-Amz-Signature=9b43013036cb90c8cc108c4619e4a0577d8b5484bee74d025656f3fe9fe14901 (which seems low) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=h1) Report
> Merging [#18083](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4375bde87afdc037056ff90e4f93b606c76e140?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18083/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18083      +/-   ##
==========================================
- Coverage   91.27%   91.23%   -0.05%     
==========================================
  Files         163      163              
  Lines       50120    50120              
==========================================
- Hits        45749    45728      -21     
- Misses       4371     4392      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.32% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/eval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXZhbC5weQ==) | `97.02% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <ø> (-0.1%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.42% <ø> (ø)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.04% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.94% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.68% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18083?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/18083?src=pr&el=footer). Last update [b4375bd...e2dfa58](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=h1) Report
> Merging [#18083](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8dac633142daa8d5bcd0cf77ad89b97628d474eb?src=pr&el=desc) will **decrease** coverage by `0.16%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18083/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18083      +/-   ##
==========================================
- Coverage   91.42%   91.26%   -0.17%     
==========================================
  Files         163      163              
  Lines       50068    50116      +48     
==========================================
- Hits        45776    45736      -40     
- Misses       4292     4380      +88
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.07% <ø> (-0.15%)` | :arrow_down: |
| #single | `40.31% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/eval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXZhbC5weQ==) | `97.02% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.94% <ø> (-0.08%)` | :arrow_down: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.68% <ø> (-0.21%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.42% <ø> (-3.31%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.02% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <ø> (-0.15%)` | :arrow_down: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `98.34% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.18% <0%> (-1.83%)` | :arrow_down: |
| ... and [17 more](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18083?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/18083?src=pr&el=footer). Last update [8dac633...c1b25e9](https://codecov.io/gh/pandas-dev/pandas/pull/18083?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 33 warnings in https://travis-ci.org/pandas-dev/pandas/jobs/296582210, making progress :) Merging this later today I think, my next round of changing removing our vendored numpydoc and ipython sphinx ext will be more invasive. Just the clipboard failure (which seems to be failing more regularly now?)

Merging. Not worth the trouble to backport, moving to 0.22.0