UBI: clarify the volume notification types' doc

I realized the new descriptions of ADDED and REMOVED could also be
misleading: they can also be triggered after using a userland util
(ubi{mk,rm}vol).

Artem: amend the commentaries

Signed-off-by David Wagner <david.wagner@free-electrons.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>