Document that objects may be deduplicated #516

Two create calls (e.g. to wgpuCreateBindGroupLayout) may return the same pointer. We should document this so that people are aware if trying to use them as map keys, or something.

I think we can specify exactly which objects may be deduplicated (instance?, bind group, bind group layout, pipeline layout, shader module, render/compute pipeline, sampler, texture view) and which can't be (adapter?, device, queue?, buffer, texture, queryset, encoders / passes / command buffers / render bundles).

See also #167