nohz: Improve a bit the full dynticks Kconfig documentation

Remove the "single task" statement from CONFIG_NO_HZ_FULL
title. The constraint can be invalidated when tasks from
other sched classes than SCHED_FAIR are running. Moreover
it's possible that hrtick join the party in the future.

Also add a line about the dependency on SMP.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

kernel/time/Kconfig

@@ -97,11 +97,13 @@ config NO_HZ_IDLE
 	  Most of the time you want to say Y here.
 
 config NO_HZ_FULL
-	bool "Full dynticks system (tickless single task)"
+	bool "Full dynticks system (tickless)"
 	# NO_HZ_COMMON dependency
 	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
-	# RCU_USER_QS
-	depends on HAVE_CONTEXT_TRACKING && SMP
+	# We need at least one periodic CPU for timekeeping
+	depends on SMP
+	# RCU_USER_QS dependency
+	depends on HAVE_CONTEXT_TRACKING
 	# RCU_NOCB_CPU dependency
 	depends on TREE_RCU || TREE_PREEMPT_RCU
 	depends on VIRT_CPU_ACCOUNTING_GEN