rcu: upgrade stallwarn.txt documentation for CPU-bound RT processes

o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel

	without invoking schedule().

o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might

	happen to preempt a low-priority task in the middle of an RCU

	read-side critical section.   This is especially damaging if

	that low-priority task is not permitted to run on any other CPU,

	in which case the next RCU grace period can never complete, which

	will eventually cause the system to run out of memory and hang.

	While the system is in the process of running itself out of

	memory, you might see stall-warning messages.

o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that

	is running at a higher priority than the RCU softirq threads.

	This will prevent RCU callbacks from ever being invoked,

	and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent

	RCU grace periods from ever completing.  Either way, the

	system will eventually run out of memory and hang.  In the

	CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning

	messages.

o	A bug in the RCU implementation.

o	A hardware failure.  This is quite unlikely, but has occurred