DOC: update the DataFrame.sub docstring
Checklist for the pandas documentation sprint :

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
################################## Validation ##################################
################################################################################

Errors found:
	Use only one blank line to separate sections or paragraphs
	Errors in parameters section
		Wrong parameters order. Actual: ('other', 'axis', 'level', 'fill_value'). Documented: ('other', 'axis', 'fill_value', 'level')
		Parameter "other" has no description
		Parameter "axis" description should finish with "."
		Parameter "fill_value" description should finish with "."
		Parameter "level" description should finish with "."
	Missing description for See Also "DataFrame.rsub" reference

```

Validation script returns docstring style errors.


I accidentally closed this PR.  Reopening # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20358?src=pr&el=h1) Report
> Merging [#20358](https://codecov.io/gh/pandas-dev/pandas/pull/20358?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a0442390038000942ae69014f77ef5faff23a068?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20358/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20358?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20358      +/-   ##
==========================================
- Coverage   91.78%   91.77%   -0.01%     
==========================================
  Files         152      152              
  Lines       49196    49176      -20     
==========================================
- Hits        45153    45133      -20     
  Misses       4043     4043
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.16% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.84% <100%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20358?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/20358/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.35% <100%> (ø)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/20358/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.17% <0%> (-0.13%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/20358/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.06% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20358/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20358/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.64% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20358?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/20358?src=pr&el=footer). Last update [a044239...4141e1e](https://codecov.io/gh/pandas-dev/pandas/pull/20358?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 When I added the "..." for the wrapped lines there were no more validation errors.
 @kurtiskerstein you seem to have included some unrelated commits. If you need any help with resolving this, let us know. Yeah.  I am going back through the  git documentation to figure it out.  I will let you know if I have further trouble.
 @kurtiskerstein Thanks for the PR!