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>
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index d7cbf7a..001accc 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -814,9 +814,8 @@
 	}
 
 	/*
-	 * Garbage Collector head likely contains long-term data and
-	 * does not need to be synchronized by timer. Also GC head nodes are
-	 * not grouped.
+	 * Garbage Collector head does not need to be synchronized by timer.
+	 * Also GC head nodes are not grouped.
 	 */
 	c->jheads[GCHD].wbuf.no_timer = 1;
 	c->jheads[GCHD].grouped = 0;