DOC: example for doing a reversed rolling window
https://github.com/pandas-dev/pandas/issues/34226#issuecomment-690887774 is the example, should just add to window.rst

also would be good to show using https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.api.indexers.FixedForwardWindowIndexer.html?highlight=forward#pandas.api.indexers.FixedForwardWindowIndexer