Blender Git Loki

Subdiv: Clarify Subdiv settings

The actual naming might also be a subject to change, especially the one
around `level`. Tricky part here is that at some point in the API there
will be change from Blender modifier's Quality to OpenSubdiv's Level,
but which API level is most suitable for this?

At least now meaning of settings is better documented ans should be
clear what's going on.