KVM: x86: explain 'no-kvmclock' kernel parameter

no-kvmclock kernel parameter is missing its explanation in
Documentation/kernel-parameters.txt. Add it.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Avi Kivity <avi@redhat.com>

Documentation/kernel-parameters.txt

@@ -1693,6 +1693,8 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	nojitter	[IA64] Disables jitter checking for ITC timers.
 
+	no-kvmclock	[X86,KVM] Disable paravirtualized KVM clock driver
+
 	nolapic		[X86-32,APIC] Do not enable or use the local APIC.
 
 	nolapic_timer	[X86-32,APIC] Do not use the local APIC timer.