UBI: clarify the volume notification types' doc
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 <47df67c0fd8cbe8b1012edad99e6694575a29d40@gmail.com>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs