landlock: Fix documentation for landlock_create_ruleset(2)

Move and fix the flags documentation, and improve formatting.

It makes more sense and it eases maintenance to document syscall flags
in landlock.h, where they are defined.  This is already the case for
landlock_restrict_self(2)'s flags.

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/20250416154716.1799902-1-mic@digikod.net


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