CLN: Follow-up comments to gh-23392
* Use 'unit' instead of 'frequency'
* Minor spacing issues in docs

Follow-up to #23392.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/dtypes/cast.py`](https://github.com/gfyoung/pandas/blob/4796ccb35406014293128ccab67244df135448a7/pandas/core/dtypes/cast.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_constructors.py`](https://github.com/gfyoung/pandas/blob/4796ccb35406014293128ccab67244df135448a7/pandas/tests/series/test_constructors.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_dtypes.py`](https://github.com/gfyoung/pandas/blob/4796ccb35406014293128ccab67244df135448a7/pandas/tests/series/test_dtypes.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23401?src=pr&el=h1) Report
> Merging [#23401](https://codecov.io/gh/pandas-dev/pandas/pull/23401?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b9e2278ef29170b051fe519d7512d0a1f011be49?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23401/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23401?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23401   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         161      161           
  Lines       51155    51155           
=======================================
  Hits        47157    47157           
  Misses       3998     3998
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (ø)` | :arrow_up: |
| #single | `42.24% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23401?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/23401/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.92% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23401?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/23401?src=pr&el=footer). Last update [b9e2278...4796ccb](https://codecov.io/gh/pandas-dev/pandas/pull/23401?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jbrockmendel is satisfied that the changes address his concerns, so merging.