DOC: DataFrame.ewm clean-up
Cosmetic change.

Thanks @sursu !

I'd mentioned this and some other changes to this docstring in #31647, do you feel like taking any of the other points too? A few people commented "take" but nobody submitted anything :) There's more there. 
For instance, `\text{ for }` shouldn't be a LaTeX text, should be normal text, otherwise in a single line we have 3 types of symbols.

I am not familiar with `:math:`, and I don't know how to preview my changes.
Does it render the entire line or only what is take in ``?

I don't understand this invention. I don't see why using `:math:` was smarter than using LaTeX/MathJax syntax. > I am not familiar with `:math:`, and I don't know how to preview my changes.
> Does it render the entire line or only what is take in ``?
> 
> I don't understand this invention. I don't see why using `:math:` was smarter than using LaTeX/MathJax syntax.

You will want to build the docs:

https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#how-to-build-the-pandas-documentation

If you can post screenshot afterwards would be helpful

We use sphinx to build restructured text. Math is a role provided therein:

https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#math Hello @sursu! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-03-07 18:39:06 UTC Screenshots:

![image](https://user-images.githubusercontent.com/12001304/75257397-2ea64f80-57e5-11ea-9703-fc5f9e5ea451.png)

![image](https://user-images.githubusercontent.com/12001304/75259306-171c9600-57e8-11ea-88ec-e4bcc132ffc2.png)

The Notes have been expanded with a few details from the given link. Ready for Review Looks like CI is red - can you take a look? Thanks @sursu , this is much nicer!

> Though, I don't know what @MarcoGorelli meant in the 5'th issue.

Just to clarify - in the doc for the respective series method (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.ewm.html), it says 

> "axis{0 or ‘index’, 1 or ‘columns’}, default 0".

 However, Series don't have columns :)

But that's a recurring issue in many parts of the documentation, so IMO it's OK to keep it separate from this Oh, so `pandas.Series.ewm.html` is also generated from `ewm.py`?

Well if someone tries to perform an EW function column-wise on a series they would bump into: 
```python
ValueError: No axis named columns for object type <class 'pandas.core.series.Series'>
```
which is a clear message.

Also, it seems that you can also use `axis=rows`. Should I specify this? ![image](https://user-images.githubusercontent.com/12001304/75488435-9c52a700-59b0-11ea-98fa-5002203fbb26.png)
 You'll have to build all the docs, not that single page to see the link.

Also, if you can merge master please, so we make the CI green and we can merge... Thanks for the good docstring @sursu 