landlock: Add design choices documentation for filesystem access rights

Summarize the rationale of filesystem access rights according to the
file type.

Update the document date.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-13-mic@digikod.net