message: little bit of docu for volume support in path tracer
diff: @@ -724,6 +724,13 @@ VolumetricModels. To create a volume instance, call
  float samplingRate 0.125 sampling rate of the volume (this
  is the minimum step size for
  adaptive sampling)
  +
  + float densityScale 1.0 used to make volumes uniformly thinner
  + or thicker ([path tracer] only)
  +
  + float anisotropy 0.0 anisotropy of the (Henyey-Greenstein)
  + phase function in [-1, 1]. Default:
  + Isotropic scattering. ([path tracer] only)
  -------------------- ------------------------ --------- ---------------------------------------
  : Parameters understood by VolumetricModel.
  @@ -1407,6 +1414,10 @@ supports the following special parameters:
  The path tracer requires that [materials] are assigned to [geometries],
  otherwise surfaces are treated as completely black.
  +The path tracer supports [volumes](#volumes) with multiple scattering. The scattering
  +albedo can be specified using the [transfer function]. Extinction is assumed
  +to be spectrally constant.
  +
  ### Materials
  Materials describe how light interacts with surfaces, they give objects
  