Document classes implementing the `GeometryFactory` interface

The GeometryFactory type is defined as if it was meant to be describing an interface. But no other class is listed as implementing the interface, so a reader of the docs has no way of knowing what classes are acceptable for use in (for example) the GeometryInstance constructor.

See this older comment for some discussion on the original intent of the GeometryFactory interface.

See https://github.com/CesiumGS/cesium/issues/12826 for an example of where a better doc for GeometryFactor could have helped avoid confusion.