Correct `PopupAnchor::PointerFixed` popups usage is not obvious

[vladbat00]

Hi! Some users of bevy_egui reached out to me about the fact that their PopupAnchor::PointerFixed popups don't work. I've never used them myself, but I was able to reproduce their issue, so I started looking into the code to understand the problem.

I found that PopupAnchor::PointerFixed popups can only be used in the combination with the open_memory command, otherwise pointer position to open the popup just won't be passed:

egui/crates/egui/src/containers/popup.rs

Line 515 in 96470fa

self.ctx.memory_mut(|mem| mem.open_popup_at(id, hover_pos));

This seems to be undocumented (but please correct me if I missed anything) and quite difficult to figure out without reading through the popup code. But it also might be a design issue, cause I'm not sure why PopupAnchor::PointerFixed popups shouldn't open when, for example, we force them to open with OpenKind::Open.

So this issue is either a documentation or a feature request.

Thanks