torture: Update RCU test scenario documentation

CONFIG_PREEMPT -- Do half.  (First three and #8.)

CONFIG_PROVE_LOCKING -- Do several, covering CONFIG_DEBUG_LOCK_ALLOC=y and not.

CONFIG_PROVE_RCU -- Hardwired to CONFIG_PROVE_LOCKING.

CONFIG_PROVE_RCU_REPEATEDLY -- Do one.

CONFIG_RCU_BOOST -- one of PREEMPT_RCU.

CONFIG_RCU_KTHREAD_PRIO -- set to 2 for _BOOST testing.

CONFIG_RCU_FANOUT -- Cover hierarchy, but overlap with others.

CONFIG_RCU_NOCB_CPU_ZERO -- Do one.

CONFIG_RCU_TRACE -- Do half.

CONFIG_SMP -- Need one !SMP for PREEMPT_RCU.

!RCU_EXPERT -- Do a few, but these have to be vanilla configurations.

CONFIG_RCU_EXPERT=n -- Do a few, but these have to be vanilla configurations.

CONFIG_RCU_EQS_DEBUG -- Do at least one for CONFIG_NO_HZ_FULL and not.

CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP -- Do for all but a couple TREE scenarios.

CONFIG_RCU_TORTURE_TEST_SLOW_INIT -- Do for all but a couple TREE scenarios.

CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT -- Do for all but a couple TREE scenarios.

RCU-bh: Do one with PREEMPT and one with !PREEMPT.

RCU-sched: Do one with PREEMPT but not BOOST.

	Always used in KVM testing.

CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT_DELAY

CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY

CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP_DELAY

	Inspection suffices, ignore.

CONFIG_PREEMPT_RCU

CONFIG_TREE_RCU

CONFIG_TINY_RCU

	These are controlled by CONFIG_PREEMPT and/or CONFIG_SMP.

CONFIG_SPARSE_RCU_POINTER

	Makes sense only for sparse runs, not for kernel builds.

CONFIG_SRCU

CONFIG_TASKS_RCU

	Selected by CONFIG_RCU_TORTURE_TEST, so cannot disable.

CONFIG_RCU_TRACE

	Implied by CONFIG_RCU_TRACE for Tree RCU.

	These are controlled by CONFIG_PREEMPT.

boot parameters ignored: TBD