docs, x86: add nox2apic back to kernel-parameters.txt

"nox2apic" was removed from kernel-parameters.txt by mistake, when
entries were sorted in alpha order (commit 0cb55ad2). But this early
parameter is still there, add it back to kernel-parameters.txt.

[ Impact: add boot parameter description ]

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Weidong Han <weidong.han@intel.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: iommu@lists.linux-foundation.org
Cc: allen.m.kay@intel.com
Cc: fenghua.yu@intel.com
LKML-Reference: <1239957736-6161-2-git-send-email-weidong.han@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -1588,6 +1588,8 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	nowb		[ARM]
 
+	nox2apic	[X86-64,APIC] Do not enable x2APIC mode.
+
 	nptcg=		[IA64] Override max number of concurrent global TLB
 			purges which is reported from either PAL_VM_SUMMARY or
 			SAL PALO.