RCU: add help text for "RCU implementation type"

This patch supplies help text for the "RCU implementation type"
kernel configuration choice.
Reported-by: NStefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

init/Kconfig

@@ -775,6 +775,14 @@ config PREEMPT_NOTIFIERS
 choice
 	prompt "RCU implementation type:"
 	default CLASSIC_RCU
+	help
+	  This allows you to choose either the classic RCU implementation
+	  that is designed for best read-side performance on non-realtime
+	  systems, or the preemptible RCU implementation for best latency
+	  on realtime systems.  Note that some kernel preemption modes
+	  will restrict your choice.
+
+	  Select the default if you are unsure.
 
 config CLASSIC_RCU
 	bool "Classic RCU"