Documentation: add xen.fifo_events kernel parameter description

The kernel boot parameter xen.fifo_events isn't listed in
Documentation/admin-guide/kernel-parameters.txt. Add it.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20201022094907.28560-6-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -5836,6 +5836,13 @@
 			After which time (jiffies) the event handling loop
 			should start to delay EOI handling. Default is 2.
 
+	xen.fifo_events=	[XEN]
+			Boolean parameter to disable using fifo event handling
+			even if available. Normally fifo event handling is
+			preferred over the 2-level event handling, as it is
+			fairer and the number of possible event channels is
+			much higher. Default is on (use fifo events).
+
 	nopv=		[X86,XEN,KVM,HYPER_V,VMWARE]
 			Disables the PV optimizations forcing the guest to run
 			as generic guest with no PV drivers. Currently support