rcu: Fix RCU's NMI documentation

It has long been the case that the architecture must call nmi_enter()
and nmi_exit() rather than irq_enter() and irq_exit() in order to
permit RCU read-side critical sections in NMIs.  Catch the documentation
up with reality.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Documentation/RCU/NMI-RCU.txt

@@ -95,7 +95,7 @@ not to return until all ongoing NMI handlers exit.  It is therefore safe
 to free up the handler's data as soon as synchronize_sched() returns.
 
 Important note: for this to work, the architecture in question must
-invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.
+invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.
 
 
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