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

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

Commit:

  8309f86cd41e ("x86/tsc: Provide 'tsc=unstable' boot parameter")

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

Document it.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Peter 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.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>