DOC: split docstring into multiple lines in excel.py
- [x] closes #13967 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry 

A follow-up on #14030.
Not sure whether this is the good way to do it - this seems too verbose.
I will add it to `parsers.py` or move it to `common.py` once it's done.

I also thought about moving the function to `Appender` to fix the length problem for good if necessary.

this is also an issue on read_csv doc string as well
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14073?src=pr) is 85.24% (diff: 100%)

> Merging [#14073](https://codecov.io/gh/pydata/pandas/pull/14073?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.02%**

``` diff
@@             master     #14073   diff @@
==========================================
  Files           139        140     +1   
  Lines         50511      50559    +48   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43071      43099    +28   
- Misses         7440       7460    +20   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [10bf721...654feea](https://codecov.io/gh/pydata/pandas/compare/10bf7213240eaa0960f50f86c7790d64d8973060...654feeadb66ea9bd281f6772b16df6962e1ac539?src=pr)
 @jreback  do u want me to move the func to `common.py`?
 yes move to `common.py`
 @jreback @jorisvandenbossche I assume this is what you're talking about? It seems okay to me.
I have limited access to Internet in these few days so I might not responding in time.
 @OXPHOS Just a question, what did you precisely change regarding to the previous version (I just don't remember exactly anymore, sorry). Sometimes adding a commit instead of amending the previous can make this more clearer.
 @jorisvandenbossche oh sorry my bad. I realized this problem but made the wrong fix. 
So I was doing [this only](https://github.com/pydata/pandas/pull/14073/commits/7dd824c19200ead89a91fbf8c49567ea900d0bfd#diff-0153a977179cfc73a2645cf0b7cd1f7fR172) for the last commit.

And now I used [this](https://github.com/pydata/pandas/pull/14073/commits/7dd824c19200ead89a91fbf8c49567ea900d0bfd#diff-0153a977179cfc73a2645cf0b7cd1f7fR101)
 @OXPHOS coming back to this, is the `spit_doc` function still needed now that you added a call to `textwrap.fill` (which already splits the long string created by `"', '".join(sorted(_NA_VALUES)`) ? Seems like you can remove this now?
 @jorisvandenbossche I updated the wrap char number and now both `parses.py` and `excel.py` uses `textwrap.fill`
 @OXPHOS Thanks a lot, looking good!
