For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
and 3D CPU particles. The new emitter is called "ring"
and it can emit either in a ring or cylinder fashion.
This adds the following properties for the emitter:
1. emission_ring_axis: the axis along which the ring/cylinder
will be constructed
2. emission_ring_radius: outer radius of the ring/cylinder
3. emission_ring_inner_radius: inner radius of the cylinder.
when set to zero, particles will emit in the full volume.
4. emission_ring_height: height of the ring/cylinder emitter.
