xen/spinlock: Document the xen_nopvspin parameter.

Which disables in the ticketlock slowpath the Xen PV optimization's.
Useful for diagnosing issues and comparing benchmarks in
over-commit CPU scenarios.
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/kernel-parameters.txt

@@ -3412,6 +3412,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				the unplug protocol
 			never -- do not unplug even if version check succeeds
 
+	xen_nopvspin	[X86,XEN]
+			Disables the ticketlock slowpath using Xen PV
+			optimizations.
+
 	xirc2ps_cs=	[NET,PCMCIA]
 			Format:
 			<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]