Document that objects may be deduplicated

[kainino0x]

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

[eliemichel]

I agree, it took me some time to realize I did not need to maintain my own descriptor -> bind group map since the WebGPU implem already does it