DOC: read_excel doc - fixed formatting and added examples
Fixes a formatting bug in the `read_excel` docs that caused a line break and bold print in list of `_NA_VALUES`. Adds examples in the `read_excel` docstring.

- [x] closes #18735 
- [x] tests added & passed
- [x] passes `git diff master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

First `pandas` PR, please let me know about anything that should be done differently.
Thank you! Thanks @JanLauGe! to close #18735 I'd like to see the following

1.  add `comment` in code to the explicit named keyword arguments to the `read_excel` function, and subsequent internal calls

2.  add `comment` to the docstring for the same

3.  add a few tests for `comment` functionality
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=h1) Report
> Merging [#18753](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/faeac49687d1ce1c3310bf365d402a4a2ab783bb?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18753/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #18753   +/-   ##
=======================================
  Coverage   91.58%   91.58%           
=======================================
  Files         150      150           
  Lines       48972    48972           
=======================================
  Hits        44851    44851           
  Misses       4121     4121
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=footer). Last update [faeac49...6afed06](https://codecov.io/gh/pandas-dev/pandas/pull/18753?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @chris-b1 ! To make sure I understand: You'd like `comment` to be a named argument instead of a keyword argument? Thanks for the comments @jreback, that should be all done now.
Please let me know if there is anything else outstanding. I spent a bunch of time lining up the arguments in read_excel to match read_csv

Would it be possible to keep following this pattern? Looks like `comment` can go between `thousands` and `skip_footer` Good point @alysivji, correct now? LGTM! uhoh, seems I mocked something up.
Sorry @jreback, can you explain to me why is AppVeyor suddenly failing now? Hello @JanLauGe! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 30, 2017 at 12:36 Hours UTC pushed a fix for the lint issue. ping on green. thanks @JanLauGe 