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>

include/linux/mtd/ubi.h

@@ -155,12 +155,14 @@ struct ubi_device_info {
 };
 
 /*
- * enum - volume notification types.
- * @UBI_VOLUME_ADDED: volume has been added
- * @UBI_VOLUME_REMOVED: start volume volume
- * @UBI_VOLUME_RESIZED: volume size has been re-sized
- * @UBI_VOLUME_RENAMED: volume name has been re-named
- * @UBI_VOLUME_UPDATED: volume name has been updated
+ * Volume notification types.
+ * @UBI_VOLUME_ADDED: a volume has been added (an UBI device was attached or a
+ *                    volume was created)
+ * @UBI_VOLUME_REMOVED: a volume has been removed (an UBI device was detached
+ *			or a volume was removed)
+ * @UBI_VOLUME_RESIZED: a volume has been re-sized
+ * @UBI_VOLUME_RENAMED: a volume has been re-named
+ * @UBI_VOLUME_UPDATED: data has been written to a volume
  *
  * These constants define which type of event has happened when a volume
  * notification function is invoked.