GPU: Clarify InstanceIndex / VertexIndex builtin behavior wrt. first_instance / first_vertex / vertex_offset?

[rabbit-ecl]

SDL_GPU can't solve the "works on my machine" problem, but it can at least explain to users exactly why they can't rely on these builtins w/ first_instance & friends and what the platform behavior is. There's no doubt going to be people asking "but why?" so might as well pre-empt that with a doc comment. See: https://github.com/gpuweb/gpuweb/issues/901