liveupdate: document systemd support

liveupdate: document systemd support

systemd v261 will add native support for LUO via the file descriptor
store mechanism. Add a brief paragraph in the LUO userspace documentation
page to inform users.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://patch.msgid.link/20260519152957.548119-1-luca.boccassi@gmail.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>