DOC, TST: Document and Test Functions in dtypes/common.py
Title is self-explanatory.

Closes #15895.
When reviewing, please note the `TODO`'s that I added along the way.  Those I think are certainly up for discussion in terms of cleaning up the API and subsequent PR's. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=h1) Report
> Merging [#16237](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2002da33b0a755fcf7ef64b2c87ca4252f0e7df0?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16237/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16237      +/-   ##
==========================================
+ Coverage   90.24%   90.24%   +<.01%     
==========================================
  Files         164      164              
  Lines       50894    50890       -4     
==========================================
- Hits        45930    45927       -3     
+ Misses       4964     4963       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.03% <100%> (+0.01%)` | :arrow_up: |
| #single | `40.3% <87.5%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.75% <100%> (+1.24%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.58% <0%> (-0.1%)` | :arrow_down: |
| [pandas/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=tree#diff-cGFuZGFzL192ZXJzaW9uLnB5) | `44.65% <0%> (+1.9%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16237?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/16237?src=pr&el=footer). Last update [2002da3...a552110](https://codecov.io/gh/pandas-dev/pandas/pull/16237?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > When reviewing, please note the TODO's that I added along the way. Those I think are certainly up for discussion in terms of cleaning up the API and subsequent PR's.

best maybe to open a separate issue for this. > best maybe to open a separate issue for this.

Sounds good. thanks!