Documentation: document liveupdate cmdline parameter

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/20260130112036.359806-1-me@linux.beauty
Signed-off-by: Li Chen <me@linux.beauty>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Pratyush Yadav <ptyadav@amazon.de>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>