update creating_an_annotation_task.md (#4231)

site/content/en/docs/manual/basics/creating_an_annotation_task.md

@@ -82,7 +82,7 @@ For more information on how to attach cloud storage, see [attach cloud storage](
 
 ### Data formats for a 3D task
 
-To create a 3D task, you need to use the following directory structures:
+To create a 3D task, you must prepare an archive with one of the following directory structures:
 {{< tabpane >}}
   {{< tab header="Velodyne" >}}
     VELODYNE FORMAT
@@ -90,7 +90,8 @@ To create a 3D task, you need to use the following directory structures:
       velodyne_points/
         data/
           image_01.bin
-    IMAGE_00 # unknown dirname, Generally image_01.png can be under IMAGE_00, IMAGE_01, IMAGE_02, IMAGE_03, etc
+          IMAGE_00 # unknown dirname,
+                   # generally image_01.png can be under IMAGE_00, IMAGE_01, IMAGE_02, IMAGE_03, etc
       data/
         image_01.png
   {{< /tab >}}
@@ -121,6 +122,8 @@ To create a 3D task, you need to use the following directory structures:
   {{< /tab >}}
 {{< /tabpane >}}
 
+> You can't mix 2D and 3D data in the same task.
+
 ## Advanced configuration
 
 ![](/images/image128.jpg)