Clarify meaning of facecolors for LineCollection

[timhoffm]

PR Summary

In response to #19168. Hopefully this is now better to understand.

[efiring]

The clarification looks good to me. I think the fundamental problem is that filling the polygons is a mistake. It is overloading LineCollection with functionality that it wasn't designed for, and that doesn't fit the name, because "we can". We could split that functionality out into a separate subclass with a better name and deprecate the filling functionality of LineCollection. This is in the spirit of @brunobeltran's effort to define the API we would like to end up with.

[timhoffm]

I agree that LineCollection should not have facecolors, and the implemented meaning is odd at best.

For now, I'm not takeing up the topic though. This is just a small PR to document the oddness in the hope that it's less confusing. I'd like to get this merged without much fuss and move on.

Others are welcome to dig into it and propose API improvements in separate PRs. For example, I'm unclear if PathCollection is sufficient or if we really want something like a FilledLineCollection subclass. It could well be that it's ok to just deprecate facecolors in LineCollection.