Clearcoat layering lerp

I have seen a few people getting stomped on the clearcoat layering logic.

Here are a few thougths:

1. This regards the doc only: lerp(t, a, b) is not typical. Osl's mix(), glsl's mix(), MDL's lerp all have the syntax as lerp(a, b, t). It's a small change in the doc but might help simplify a bit.

2. if coat_affect_color default to 0.0 and is "optional", then coat_color should be optional too, because in the simplified model it will have no effect at all and confuse users.

3. I realize it might be late for this, but the name coat_color is a bit misleading as most of the time a color associated to a lobe should affect the lobe, while here it's affecting everything BUT the lobe. "coat_filter_color" or something more intuitive could have helped.