[WebGPU] Document Separate read-only state for stencil and depth aspects

[chrisdavidmills]

See https://developer.chrome.com/blog/new-in-webgpu-123#separate_read-only_state_for_stencil_and_depth_aspects for change description.

Acceptance criteria

• Add a description of this change to https://developer.mozilla.org/en-US/docs/Web/API/GPUCommandEncoder/beginRenderPass#depthstencil_attachment_object_structure, somewhere around the depthReadOnly and stencilReadOnly properties? Or maybe the BCD will be enough?
• Add a BCD data point for this addition — this shipped in Chrome 123.