Update user_guide.md, images and gif (#1556)

Co-authored-by: Nikita Manovich <40690625+nmanovic@users.noreply.github.com>

cvat/apps/documentation/user_guide.md

@@ -22,7 +22,7 @@
       - [Objects](#objects)
       - [Labels](#labels)
   - [Shape mode (advanced)](#shape-mode-advanced)
-  - [Interpolation mode (advanced)](#interpolation-mode-advanced)
+  - [Track mode (advanced)](#track-mode-advanced)
   - [Attribute annotation mode (advanced)](#attribute-annotation-mode-advanced)
   - [Annotation with rectangle by 4 points](#annotation-with-rectangle-by-4-points)
   - [Annotation with polygons](#annotation-with-polygons)
@@ -984,19 +984,23 @@ by accident. Shortcut: ``L``.
 
 ![](static/documentation/images/image066.jpg)
 
-## Interpolation mode (advanced)
+## Track mode (advanced)
 
-Basic operations in the mode were described above.
+Basic operations in the mode were described in section [track mode (basics)](#track-mode-basics).
 
-Bounding boxes that were created in the mode, have extra navigation buttons.
+Shapes that were created in the track mode, have extra navigation buttons.
--   These buttons help to jump to previous/next key frame.
+-   These buttons help to jump to the previous/next keyframe.
 
     ![](static/documentation/images/image056.jpg)
 
--   The button helps to jump to initial frame for the object (first bounding box for the track).
+-   The button helps to jump to the initial frame and to the last keyframe.
 
     ![](static/documentation/images/image057.jpg)
 
+You can use the `` Split '' function to split one track into two tracks:
+
+![](static/documentation/images/gif010_DETRAC.gif)
+
 ## Attribute annotation mode (advanced)
 
 Basic operations in the mode were described in section [attribute annotation mode (basics)](#attribute-annotation-mode-basics).