Document winding order of Surface rings as output by surfaces_as_geojson

The topo_update.surfaces_as_geojson function does not mention any specific winding order for rings of Surfaces as written in the geometry object of the GeoJSON nor as written in the geometry_properties object.

Reading the code it looks like the driving force of the geometry_properties object is the topo_update.get_surface_borders function, which is documented to use a sign as a way to know how to keep the Surface area on the left side of the walking direction, which looks to me as meaning shells should be counter-clockwise, but a more explicit description of this is not found in the outermost topo_update.surfaces_as_geojson method.

See ar5web_pgtopo_update_sql#19 (closed) and see also #253 (closed)