Fix disparity between docs and testrender for oren_nayar() closure.

The docs described it as "orennayar", but it somehow was never included in stdosl.h, and when we implemented it for "testrender", it was called "oren_nayar."

I'm sticking with the "oren_nayar" name and fixing the rest to conform. Also, in the docs, changing the parameter name from "roughness" to "sigma" and fixing a few other typos where we referred to a "lambert" BSDF that's actually just called "diffuse".