Bidets, specifically the “bidet showers” that are commonly nicknamed bum guns in English (as opposed to Arab-style bidet bowls). I prefer going to restrooms where such exists for the sake of convenience.

Admittedly, it's very much undocumented, but since it's a niche enough feature, the only discussion or any sort of tagging scheme at all that I could find on either the wiki or the forum is this one from @AkuAnakTimur:

https://community.openstreetmap.org/t/water-in-toilets/84863/2