Fix  excel-related docstring errors 
- [x] closes #23494
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] passes/runs `python make.py --single pandas.related-doc`

Fix errors which are  reported by `scripts/validate_docstrings.py`:
 
 * pandas.read_excel
 * pandas.io.formats.style.Styler.to_excel
 * Series.to_excel
 * DataFrame.to_excel

@datapythonista @WillAyd @gfyoung if any comments. `python make.py --single pandas.read_excel` stops building docs after this commit 51c508c (merge upstream) @thoo, @datapythonista just changed this a bit, can you post what happens? I got errors like `/Users/johndoe/pandas/doc/source/index.rst:110: WARNING: toctree references excluded document 'generated/pandas.read_excel'` when I run `python make.py --single pandas.read_excel`. 
I think the extensions are not built. When I run `python setup.py build_ext --inplace -j 4`, I just get `running build_ext` and no other messages/output as usual. 

I just submitted the issue here #24505. Please let me know if you need any other info. 
 @thoo I just built `pandas_excel` on master (in Linux):
```
$ ./doc/make.py html --single=pandas.read_excel
Running Sphinx v1.8.2
making output directory...
[autosummary] generating autosummary for: index.rst
[autosummary] generating autosummary for: /home/mgarcia/src/pandas/doc/source/generated/pandas.read_excel.rst
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1 source files that are out of date
updating environment: 2 added, 0 changed, 0 removed
reading sources... [100%] index                                                                                                                                                                                                               
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index                                                                                                                                                                                                                
generating indices... genindex py-modindex
writing additional pages... search
copying static files... done
copying extra files... done
dumping search index in English (code: en) ... done
dumping object inventory... done
build succeeded.

The HTML pages are in doc/build/html.
```

I think we do a clean before the build, and if I'm not wrong I left the clean of the `doc/source/generated/` directory, but may be you can try to remove it manually if it exists. Ups, sorry, my bad. I didn't have the last changes, looks like building a single API doc is broken after splitting `api.rst` that has been just merged. Let me take a look... @datapythonista Thanks. I will remove `doc/source/generated/`.  Opened #24506, which should fix the problem. @thoo if you'd pull from master, you should be able to build single pages now. @jreback @datapythonista  Thanks. It is working.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=h1) Report
> Merging [#24503](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4ac0d68b2418e261f2fd8f3a9f8eca0d5e20474?src=pr&el=desc) will **decrease** coverage by `49.25%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24503/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24503       +/-   ##
===========================================
- Coverage   92.31%   43.05%   -49.26%     
===========================================
  Files         166      166               
  Lines       52391    52391               
===========================================
- Hits        48363    22557    -25806     
- Misses       4028    29834    +25806
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `43.05% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `39.83% <ø> (-56.79%)` | :arrow_down: |
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-98.65%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| ... and [125 more](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24503?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/24503?src=pr&el=footer). Last update [c4ac0d6...f11bce8](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=h1) Report
> Merging [#24503](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/100ffffa0ace51baac7045ca51fecf228e54bc5b?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24503/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24503      +/-   ##
==========================================
- Coverage   31.89%   31.88%   -0.02%     
==========================================
  Files         166      166              
  Lines       52421    52434      +13     
==========================================
- Hits        16722    16718       -4     
- Misses      35699    35716      +17
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.28% <ø> (-0.01%)` | :arrow_down: |
| #single | `31.88% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `31.41% <ø> (ø)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `38.09% <0%> (-0.4%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `27.82% <0%> (-0.14%)` | :arrow_down: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `7.97% <0%> (-0.09%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `8.77% <0%> (ø)` | :arrow_up: |
| [pandas/core/computation/expressions.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwcmVzc2lvbnMucHk=) | `58.82% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `23.65% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `13.71% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `29.81% <0%> (+0.12%)` | :arrow_up: |
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24503/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `27.89% <0%> (+0.29%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24503?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/24503?src=pr&el=footer). Last update [100ffff...be9651e](https://codecov.io/gh/pandas-dev/pandas/pull/24503?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 It looks like  `pytest pandas/tests/io/test_excel.py`  doesn't pass on master. So hopefully that is not related to this PR.  there are some tests that check the content of the docstrings, so it's probably related to it, I'd take a look thanks @thoo 