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.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>