UBI: improve comment

Explain better the purpose of thie 'move_to_put' stuff.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/wl.c

@@ -1139,8 +1139,11 @@ int ubi_wl_put_peb(struct ubi_device *ubi, int pnum, int torture)
 		/*
 		 * User is putting the physical eraseblock which was selected
 		 * as the target the data is moved to. It may happen if the EBA
-		 * unit already re-mapped the LEB but the WL unit did has not
-		 * put the PEB to the "used" tree.
+		 * unit already re-mapped the LEB in 'ubi_eba_copy_leb()' but
+		 * the WL unit has not put the PEB to the "used" tree yet, but
+		 * it is about to do this. So we just set a flag which will
+		 * tell the WL worker that the PEB is not needed anymore and
+		 * should be sheduled for erasure.
 		 */
 		dbg_wl("PEB %d is the target of data moving", pnum);
 		ubi_assert(!ubi->move_to_put);