Clarify GeoJSONL spec

[mvexel]

Our spec is different from the various "specs" that live out there on the internet. Note that there is no official GeoJSONL spec, but that's all the more reason we should be clear about ours.

See https://osmus.slack.com/archives/C04M1NHPEQY/p1740551156419729

I think https://learn.maproulette.org/en-us/documentation/line-by-line-geojson/#content is pretty clear but if there is room for improvement we should consider it.