Improve documentation and error message to clarify user defined data structures: `struct` vs `class`

[chaoticbob]

It appears that Slang does not support a class data structure? I couldn't find an example of a class in the tests. When I try to compile the shader below, I get this error:

(0): error 99999: Slang compilation aborted due to an exception of class Slang::InternalError: unimplemented: Unhandled global inst in spirv-emit:
[export(%1)]
[nameHint(%2)]
class %S        : %3
{
        field(%a, %4)
}

CMD:

slangc.exe -target spirv -lang slang -D__spirv__ -emit-spirv-directly -profile vs_6_0 -entry main shader.hlsl

If class isn't supported and the user is meant to use struct, can we document this instead of leaving it as an exercise for the the user?

Shader

class S {  
  float a;
};

float4 main() : SV_TARGET {
    S s;

    return (float4)s.a;
}

[csyonghe]

class is reserved for further cpu/host side logic, and represents a reference counted, heap allocated object. It is invalid to use classes for gpu code.