min_periods_r refactored

unfortunately some refactoring in this commit [1], lead to renaming of min_periods to min_periods_r in much more files that actually needed. So the docstrings are wrong now.

i personally dont like the _r at all and IMO its also not necessary, because there is no other parameter without the _r, but because @schaefed approved it, i assume that i'm outvoted 🤷