UsdPreviewSurface: clarify clearcoat layering

[hybridherbst]

Description of Issue

Clearcoat and clearcoatRoughness are described here: https://openusd.org/release/spec_usdpreviewsurface.html
However, it's not immediately clear (pun intended) if clearcoat is to be applied as separate layer outside the normal map (a coating) or as a layer directly on top of the already provided normal. I think it's intended as the latter, which the description "second specular lobe" is probably meant to convey, but it could be more apparent.

For reference, glTF provides a separate clearcoatNormalMap to better define the intended effect (when both normals are the same the behaviour is the same as in UsdPreviewSurface right now).

.glb and .usdz files for comparison:
ClearCoatTest.zip

[jesschimein]

Filed as internal issue #USD-8665