DOC: reverse rolling window (#38627)
This adds the suggested changes from closed pull request (#38627):
https://github.com/pandas-dev/pandas/pull/39091/files/80819f65402e46caa29ff7192b34d43f91c1a0ee
After updated commit.
- remove noqa
- change line back to single quotes
- rename `df1` to `reversed_df`
- add reference to `FixedForwardWindowIndexer`
- remove repeated `df` initialization in code example
- rename `df2` to `reversed_df`
- move code example

- [x] closes #38627
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

> Could you just add the new section and save cleanups for another PR (so just add content)?
> 
> It still appears that things changed that were not supposed to in the PR.

Yeah, I can do that. I realized I accidentally deleted part of a code example.

```
For some problems...
.. ipython:: python

   from pandas.api.indexers import FixedForwardWindowIndexer
   indexer = FixedForwardWindowIndexer(window_size=2)
   df.rolling(indexer, min_periods=1).sum()
``` Updated, does this work now? Thanks @mdhsieh. CI failure unrelated 