Discuss spherical projections used in docs

The S2 library as far as I understand is concerned only with geometry on the unit sphere and is agnostic as to how one arrives there from the 3D geometry that is the earth. It's fair that the docs do not go into detail (even though it would be nice), but since they contain [images of S2 cells on the earth](http://s2geometry.io/) and [a list of cell sizes in km²](http://s2geometry.io/resources/s2cell_statistics), I wish they would mention which spherical projection was used for those. Is it just a radial projection, or are 3D points converted to Lat/Lon/Alt first and reinterpreted as Lat/Lon on the sphere?

By the way, searching for "spherical projections" mostly turns up articles about projecting a sphere onto the plane, so a link to some further reading would also be appreciated.