DOC: Update pct_change documentation
Related to SO question: [How to avoid bfill or ffill when calculating pct_change with NaNs](https://stackoverflow.com/questions/75395345/how-to-avoid-bfill-or-ffill-when-calculating-pct-change-with-nans)

Thanks @corralien  Thanks @corralien for having fixed this and brought it to our attention!

It actually gets slightly worse: from http://pandas-coverage.herokuapp.com/ it looks like there's no test case with `fill_method` as `None`, and the type hints don't even cover it. I'll open a new issue about 