Mention NaN handling in dtype description
To achieve preservation and avoid interpretation of string or object dtypes, NaN value interpretation must be switched off.

- [x] closes issue #20875
- [ ] tests passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] `pandas.read_csv.html` renders text as intended

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20895?src=pr&el=h1) Report
> Merging [#20895](https://codecov.io/gh/pandas-dev/pandas/pull/20895?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/28edd06498987972953b5c8e1c98b7f55d24b8d2?src=pr&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20895/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20895?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20895      +/-   ##
==========================================
+ Coverage   91.78%   91.82%   +0.04%     
==========================================
  Files         153      153              
  Lines       49341    49491     +150     
==========================================
+ Hits        45287    45446     +159     
+ Misses       4054     4045       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <ø> (+0.04%)` | :arrow_up: |
| #single | `41.85% <ø> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20895?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <ø> (ø)` | :arrow_up: |
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `91.95% <0%> (-0.99%)` | :arrow_down: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.27% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.02% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.97% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.08% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.29% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.25% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <0%> (ø)` | :arrow_up: |
| ... and [14 more](https://codecov.io/gh/pandas-dev/pandas/pull/20895/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20895?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/20895?src=pr&el=footer). Last update [28edd06...53dd911](https://codecov.io/gh/pandas-dev/pandas/pull/20895?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 CircleCI failures is fixed on master.

Thanks @jowagner. And thanks for your patience while I learned how to do this.