Document `polygonToCells` limitation of 180 degree width

[nrabinowitz]

The polyfill/polygonToCells function currently assumes that any polygon with an arc that's > 180 degrees of longitude crosses the antimeridian, leading to problems when users try to fill a world-size bounding box (e.g. a web map viewport). We should document this restriction and possible workarounds (inserting dummy vertices, splitting the polygon).