[WebGPU] Document validation error criteria: Submitted command buffers must be unique

[chrisdavidmills]

See https://developer.chrome.com/blog/new-in-webgpu-126#submitted_command_buffers_must_be_unique for change description.

Acceptance criteria

• Update the commandBuffers description on https://developer.mozilla.org/en-US/docs/Web/API/GPUQueue/submit to say that they must be unique.
• Update the Validation section on https://developer.mozilla.org/en-US/docs/Web/API/GPUQueue/submit to add a description of this validation error criterium.
• Add a BCD data point for these changes — this shipped in Chrome 126.