x86, documentation: nmi_watchdog=2 works on x86_64

Signed-off-by: NMarcin Slusarz <marcin.slusarz@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/nmi_watchdog.txt

@@ -23,8 +23,7 @@ kernel debugging options, such as Kernel Stack Meter or Kernel Tracer,
 may implicitly disable the NMI watchdog.]
 
 For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
-always enabled with I/O-APIC mode (nmi_watchdog=1). Currently, local APIC
-mode (nmi_watchdog=2) does not work on x86-64.
+always enabled with I/O-APIC mode (nmi_watchdog=1).
 
 Using local APIC (nmi_watchdog=2) needs the first performance register, so
 you can't use it for other purposes (such as high precision performance