landlock: Update log documentation

Fix and improve documentation related to landlock_restrict_self(2)'s
flags.  Update the LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF
documentation according to the current semantic.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250416154716.1799902-3-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>