UBI: amend commentaries WRT dtype

Richard removed the "dtype" hint, but few commentaries were left and this patch
removes them. I've also added a better description about the "dtype" field in
the ubi-user.h for people who may ever wonder what was that dtype thing about.

This patch also adds an important note that it is better to use value "3" for
the "dtype" field.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>