x86/tsc: Append the 'tsc=' description for the 'tsc=unstable' boot parameter

Commit:

  8309f86c ("x86/tsc: Provide 'tsc=unstable' boot parameter")

added a new 'tsc=unstable' parameter, but didn't document it.

Document it.
Signed-off-by: NDou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: <corbet@lwn.net>
Cc: <pasha.tatashin@oracle.com>
Cc: <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1507539813-11420-1-git-send-email-douly.fnst@cn.fujitsu.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4194,6 +4194,9 @@
 			Used to run time disable IRQ_TIME_ACCOUNTING on any
 			platforms where RDTSC is slow and this accounting
 			can add overhead.
+			[x86] unstable: mark the TSC clocksource as unstable, this
+			marks the TSC unconditionally unstable at bootup and
+			avoids any further wobbles once the TSC watchdog notices.
 
 	turbografx.map[2|3]=	[HW,JOY]
 			TurboGraFX parallel port interface