Document VertexAttribute and VertexBinding

[TheSpydog]

It's not immediately clear how vertex bindings and attributes relate to each other, and the names of the struct members doesn't elucidate much. Some feedback:

Both have a Uint32 binding; field; I'm guessing the one in SDL_GpuVertexAttribute refers to a SDL_GpuVertexBinding and the one in SDL_GpuVertexBinding to a bound vertex buffer… maybe?

We should add some high-level documentation to the header and potentially rename the binding field of at least one of these structs.