DOC: further clean-up null/na changes
Old whatsnew example was failing -> converted to static code-block

(+ some additional minor fixes)
> Old whatsnew example was failing -> converted to static code-block

Just curious, do you know why it was failing? > Just curious, do you know why it was failing?

Ah, good question. As it should be supposed to raise a warning instead of raise I think? (that's at least what it says in the whatsnew notice)

 @jreback 

```
In [24]: pd.set_option('use_inf_as_null', True)
...
OptionError: "No such keys(s): 'use_inf_as_null'"

In [25]: pd.options.mode.use_inf_as_null
...
KeyError: 'use_inf_as_null'
```

those should issue a warning instead? Opened https://github.com/pandas-dev/pandas/issues/17115 to keep track of this # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=h1) Report
> Merging [#17113](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/465c59f964c8d71d8bedd16fcaa00e4328177cb1?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/17113/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17113      +/-   ##
==========================================
+ Coverage   91.02%   91.03%   +<.01%     
==========================================
  Files         161      161              
  Lines       49397    49397              
==========================================
+ Hits        44966    44969       +3     
+ Misses       4431     4428       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.8% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.28% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `95.9% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17113?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/17113?src=pr&el=footer). Last update [465c59f...cdd1a62](https://codecov.io/gh/pandas-dev/pandas/pull/17113?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
