landlock: Fix landlock_add_rule(2) documentation

It is not mandatory to pass a file descriptor obtained with the O_PATH
flag.  Also, replace rule's accesses with ruleset's accesses.

Link: https://lore.kernel.org/r/20220506160820.524344-2-mic@digikod.net


Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>