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

Update the document date.
Reviewed-by: NPaul Moore <paul@paul-moore.com>
Signed-off-by: NMickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-13-mic@digikod.net