documented preserve_order in Geom.line

I didn't realize this was possible at first. I'll probably use it for drawing geographic outlines.

FWIW, I was looking for this initially as `Geom.path`. I wonder if we want a `Geom.path()` that just creates a `Geom.LineGeometry` with `preserve_order=true`? If you think so, I could make a PR adding that.