DOC: Spellcheck of categorical.rst and visualization.rst
Minor changes to the documentation, specifically `categorical.rst` and `visualization.rst`:

* Function references as links.
* Backticks ` `` ` around Series, DataFrame.
* Minor rephrasing of sentences, spelling, etc.
* For plots such as lag plots, Andrews plot, Radviz, I added links to Wikipedia entries.

Comments very welcome.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19428?src=pr&el=h1) Report
> Merging [#19428](https://codecov.io/gh/pandas-dev/pandas/pull/19428?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12ac43f98c6d3ce9c0c48ed75b80346881c1e4d4?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/19428/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19428?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19428   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48728    48728           
=======================================
  Hits        44645    44645           
  Misses       4083     4083
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19428?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/19428?src=pr&el=footer). Last update [12ac43f...59065c7](https://codecov.io/gh/pandas-dev/pandas/pull/19428?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @tommyod . these are *very* welcome PR's. If possible, can you put a PR per file (unless very little changes). Just a bit easier to review. thanks! keep em coming @jreback I expanded the reference by writing a sentence about the `cut` function. I hope I understood your comment correctly. I believe this PR is good to go.

**EDIT:** Updated the page and realized that this was merged. Thanks @jreback ! I can do 1 file per PR, no problem.