[WebGPU] Document the ability to unset bind groups by passing null

[chrisdavidmills]

See https://developer.chrome.com/blog/new-in-webgpu-117#unset_bind_group for change description.

Note that the above link does not mention GPUComputePassEncoder/setBindGroup, but I tested it and it works there as well.

Acceptance criteria

• Update the description of bindGroup at https://developer.mozilla.org/en-US/docs/Web/API/GPUComputePassEncoder/setBindGroup#bindgroup, https://developer.mozilla.org/en-US/docs/Web/API/GPURenderPassEncoder/setBindGroup#bindgroup, and https://developer.mozilla.org/en-US/docs/Web/API/GPURenderBundleEncoder/setBindGroup#bindgroup to say that null can be passed to unset a previously-set bind group in a particular slot. Use the same kind of text as https://developer.mozilla.org/en-US/docs/Web/API/GPURenderPassEncoder/setVertexBuffer#buffer.
• Add unsetting examples to the pages linked above, in the same style as https://developer.mozilla.org/en-US/docs/Web/API/GPURenderPassEncoder/setVertexBuffer#unset_vertex_buffer
• Add BCD data points for these changes, to GPUComputePassEncoder.setBindGroup, GPURenderPassEncoder.setBindGroup, and GPURenderBundleEncoder.setBindGroup. They shipped in Chrome 117.

[chrisdavidmills]

Content PR: #36648.

BCD PR: mdn/browser-compat-data#24936