Documentation: document liveupdate cmdline parameter

liveupdate is used to enable Live Update Orchestrator (LUO) early during
boot. Add it to kernel-parameters.txt so users can discover and use it.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Li Chen <[e-mail]>
Acked-by: Mike Rapoport (Microsoft) <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: "Borislav Petkov (AMD)" <[e-mail]>
Cc: Frank van der Linden <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Li RongQing <[e-mail]>
Cc: Pawan Gupta <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Pratyush Yadav <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			If there are multiple matching configurations changing
 			the same attribute, the last one is used.
 
+	liveupdate=	[KNL,EARLY]
+			Format: <bool>
+			Enable Live Update Orchestrator (LUO).
+			Default: off.
+
 	load_ramdisk=	[RAM] [Deprecated]
 
 	lockd.nlm_grace_period=P  [NFS] Assign grace period.