samples/landlock: Clarify option parsing behaviour

samples/landlock: Clarify option parsing behaviour

Clarify the distinction between filesystem variables (mandatory)
and all others (optional).

For optional variables, explain the difference between unset variables
(no access check performed) and empty variables (nothing allowed for
lists of allowed paths/ports, or no effect for lists of scopes).

List the known LL_SCOPED values and their effect.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://lore.kernel.org/r/20241019151534.1400605-4-matthieu@buffet.re
[mic: Add a missing colon]
Signed-off-by: Mickaël Salaün <mic@digikod.net>