docs: clarify oneDNN backend coverage

[zcxGGmu]

Summary

• Clarify that the pre-built oneDNN EP package is CPU-oriented.
• Document the source-build path for enabling oneDNN's OpenCL GPU runtime with --dnnl_gpu_runtime ocl.
• Note that GPU coverage depends on the underlying oneDNN GPU implementation and ORT oneDNN EP operator support.

Fixes #22971

Test Plan

• python focused check for the oneDNN backend coverage note
• source grep for --dnnl_gpu_runtime and --dnnl_opencl_root build arguments
• git diff --check
• diff secret/conflict scan

[azure-pipelines]

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