doc: Document rcuperf's module parameters

			      reboot_cpu is s[mp]#### with #### being the processor

					to be used for rebooting.

	refperf.holdoff= [KNL]

			Set test-start holdoff period.  The purpose of

			this parameter is to delay the start of the

			test until boot completes in order to avoid

			interference.

	refperf.loops= [KNL]

			Set the number of loops over the synchronization

			primitive under test.  Increasing this number

			reduces noise due to loop start/end overhead,

			but the default has already reduced the per-pass

			noise to a handful of picoseconds on ca. 2020

			x86 laptops.

	refperf.nreaders= [KNL]

			Set number of readers.  The default value of -1

			selects N, where N is roughly 75% of the number

			of CPUs.  A value of zero is an interesting choice.

	refperf.nruns= [KNL]

			Set number of runs, each of which is dumped onto

			the console log.

	refperf.readdelay= [KNL]

			Set the read-side critical-section duration,

			measured in microseconds.

	refperf.shutdown= [KNL]

			Shut down the system at the end of the performance

			test.  This defaults to 1 (shut it down) when

			rcuperf is built into the kernel and to 0 (leave

			it running) when rcuperf is built as a module.

	refperf.verbose= [KNL]

			Enable additional printk() statements.

	relax_domain_level=

			[KNL, SMP] Set scheduler's default relax_domain_level.

			See Documentation/admin-guide/cgroup-v1/cpusets.rst.