Documenting ol.Sphere

[probins]

As discussed in #2328 (comment) ol.Sphere is used in the Tissot example but doesn't appear in the docs, which aren't very helpful in this respect. Adding @api to ol.Sphere makes it appear in the docs, and provides a link from ol.geom.Polygon#circular, but this doesn't help with ol.Sphere.WGS84, which doesn't appear even though it is @api. This is because it isn't a subclass but an instance of ol.Sphere, defined in the tags as a @const. JSDoc doesn't know what to do with these.

As ol.Sphere.WGS84 (and ol.Sphere.NORMAL) simply provides a radius to ol.Sphere, I'm wondering if it wouldn't be better to have these radii stored in an enum or something rather than as separate symbols.