Clarify cl_khr_kernel_clock vs. event profiling

Clarify in the extension spec that the means of conveying time-dimensional meaning to the values returned by kernel clock is implementation/vendor specific. It is not guaranteed to coincide with values measured on the host by event profiling, even if those are documented to depend on timers running on the device.

Start with the cl_khr_kernel_clock extension spec and its discussion of event profiling. Clarify that the time-dimensional meaning of kernel-clock values is implementation/vendor specific and need not match host event-profiling measurements, even when device timers are involved; the spec text should make this distinction explicit.

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