DOC: clean-up recent doc errors/warnings
Fixing some sphinx errors / warnings.

We really should revive the PR to let travis fail on new warnings/errors...
Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/jorisvandenbossche/pandas/blob/1e5c8d5e855cf4c27323e31d9da194136b6a203e/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/jorisvandenbossche/pandas/blob/1e5c8d5e855cf4c27323e31d9da194136b6a203e/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/jorisvandenbossche/pandas/blob/1e5c8d5e855cf4c27323e31d9da194136b6a203e/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/tools/timedeltas.py`](https://github.com/jorisvandenbossche/pandas/blob/1e5c8d5e855cf4c27323e31d9da194136b6a203e/pandas/core/tools/timedeltas.py) !

 > We really should revive the PR to let travis fail on new warnings/errors...

Will revive https://github.com/pandas-dev/pandas/pull/22743 after this is in :) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23636?src=pr&el=h1) Report
> Merging [#23636](https://codecov.io/gh/pandas-dev/pandas/pull/23636?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/eb19fe94966572dad0a0b89e499ff4b78d74a519?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23636/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23636?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23636   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51326    51326           
=======================================
  Hits        47347    47347           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23636?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23636/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.01% <ø> (ø)` | :arrow_up: |
| [pandas/core/tools/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23636/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvdGltZWRlbHRhcy5weQ==) | `98.03% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23636/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.03% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23636/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.81% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23636?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/23636?src=pr&el=footer). Last update [eb19fe9...4d0be5c](https://codecov.io/gh/pandas-dev/pandas/pull/23636?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
