rcu: document ways of stalling updates in low-memory situations

	include:

	a.	Keeping a count of the number of data-structure elements

		used by the RCU-protected data structure, including those

		waiting for a grace period to elapse.  Enforce a limit

		on this number, stalling updates as needed to allow

		previously deferred frees to complete.

		Alternatively, limit only the number awaiting deferred

		free rather than the total number of elements.

		used by the RCU-protected data structure, including

		those waiting for a grace period to elapse.  Enforce a

		limit on this number, stalling updates as needed to allow

		previously deferred frees to complete.	Alternatively,

		limit only the number awaiting deferred free rather than

		the total number of elements.

		One way to stall the updates is to acquire the update-side

		mutex.	(Don't try this with a spinlock -- other CPUs

		spinning on the lock could prevent the grace period

		from ever ending.)  Another way to stall the updates

		is for the updates to use a wrapper function around

		the memory allocator, so that this wrapper function

		simulates OOM when there is too much memory awaiting an

		RCU grace period.  There are of course many other

		variations on this theme.

	b.	Limiting update rate.  For example, if updates occur only

		once per hour, then no explicit rate limiting is required,