liveupdate: document liveupdate=on

While the liveupdate= parameter is documented in kernel-parameters.txt,
it is not listed in LUO's user facing documentation. This can make it
hard for users to figure out how to enable the subsystem, since enabling
just the config isn't enough.

Note the need for the kernel parameter in LUO core documentation, which
gets exported to Documentation/core-api/liveupdate.rst.

Suggested-by: Luca Boccassi <luca.boccassi@gmail.com>
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Link: https://patch.msgid.link/20260519125714.2435640-1-pratyush@kernel.org
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: prati0100

kernel/liveupdate/luo_core.c

@@ -36,6 +36,10 @@
  *
  * LUO uses Kexec Handover to transfer memory state from the current kernel to
  * the next kernel. For more details see Documentation/core-api/kho/index.rst.
+ *
+ * .. note::
+ *     To enable LUO, boot the kernel with the ``liveupdate=on`` command line
+ *     parameter.
  */
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt