DOC: Added examples for union_categoricals
 - [x] closes #16390 
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

Copied and posted over examples into the doc string; added link.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=h1) Report
> Merging [#16397](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f55de1b334131dd0577b91eb061dcb8bc65ef78?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16397/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16397      +/-   ##
==========================================
- Coverage   90.41%   90.38%   -0.03%     
==========================================
  Files         161      161              
  Lines       50997    50999       +2     
==========================================
- Hits        46107    46097      -10     
- Misses       4890     4902      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.22% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.19% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.07% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `81.89% <0%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16397?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/16397?src=pr&el=footer). Last update [0f55de1...925a1ce](https://codecov.io/gh/pandas-dev/pandas/pull/16397?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @GuessWhoSamFoo Thanks!