Tweaked RenderManShader coshader array element naming.

The element names are now derived from the parameter name, and are sufficed with indices starting from 0 rather than 1.
Added documentation recommending numeric indexing for ArrayPlug.

I'm not particularly happy with the way the element plug naming works right now, but until we have further use cases to inform any decision making I'd like to leave it as-is. The documentation is intended to encourage users not to become dependent upon the existing naming scheme.