trace: Update documentation for mono, mono_raw and boot clock

		  to correlate events across hypervisor/guest if

		  tb_offset is known.

	  mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)

		which is monotonic and is subject to NTP rate adjustments.

	  mono_raw:

		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)

		which is montonic but is not subject to any rate adjustments

		and ticks at the same rate as the hardware clocksource.

	  boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the

		fast monotonic clock, but also accounts for time spent in

		suspend. Since the clock access is designed for use in

		tracing in the suspend path, some side effects are possible

		if clock is accessed after the suspend time is accounted before

		the fast mono clock is updated. In this case, the clock update

		appears to happen slightly sooner than it normally would have.

		Also on 32-bit systems, it's possible that the 64-bit boot offset

		sees a partial update. These effects are rare and post

		processing should be able to handle them. See comments in the

		ktime_get_boot_fast_ns() function for more information.

	To set a clock, simply echo the clock name into this file.

	  echo global > trace_clock