landlock: Fix documentation for landlock_restrict_self(2)

Fix, deduplicate, and improve rendering of landlock_restrict_self(2)'s
flags documentation.

The flags are now rendered like the syscall's parameters and
description.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250416154716.1799902-2-mic@digikod.net


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