rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

If you set RCU_FANOUT_LEAF too high, you can get lock contention
on the leaf rcu_node, and you should boot with the skew_tick kernel
parameter set in order to avoid this lock contention.  This commit
therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state
this.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

init/Kconfig

@@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
 	  initialization.  These systems tend to run CPU-bound, and thus
 	  are not helped by synchronized interrupts, and thus tend to
 	  skew them, which reduces lock contention enough that large
-	  leaf-level fanouts work well.
+	  leaf-level fanouts work well.  That said, setting leaf-level
+	  fanout to a large number will likely cause problematic
+	  lock contention on the leaf-level rcu_node structures unless
+	  you boot with the skew_tick kernel parameter.
 
 	  Select a specific number if testing RCU itself.
 
-	  Select the maximum permissible value for large systems.
+	  Select the maximum permissible value for large systems, but
+	  please understand that you may also need to set the skew_tick
+	  kernel boot parameter to avoid contention on the rcu_node
+	  structure's locks.
 
 	  Take the default if unsure.