doc: Make reader aware of rcu_dereference_protected

	unnecessary overhead on Alpha CPUs.

	Note that the value returned by rcu_dereference() is valid

	only within the enclosing RCU read-side critical section.

	only within the enclosing RCU read-side critical section [1].

	For example, the following is -not- legal:

		rcu_read_lock();

	typically used indirectly, via the _rcu list-manipulation

	primitives, such as list_for_each_entry_rcu().

	[1] The variant rcu_dereference_protected() can be used outside

	of an RCU read-side critical section as long as the usage is

	protected by locks acquired by the update-side code.  This variant

	avoids the lockdep warning that would happen when using (for

	example) rcu_dereference() without rcu_read_lock() protection.

	Using rcu_dereference_protected() also has the advantage

	of permitting compiler optimizations that rcu_dereference()

	must prohibit.	The rcu_dereference_protected() variant takes

	a lockdep expression to indicate which locks must be acquired

	by the caller. If the indicated protection is not provided,

	a lockdep splat is emitted.  See RCU/Design/Requirements.html

	and the API's code comments for more details and example usage.

The following diagram shows how each API communicates among the

reader, updater, and reclaimer.