Update item "Basic navigation" in User Guide and bug fix in user_guide.md

cvat/apps/documentation/user_guide.md

@@ -187,7 +187,7 @@ Go to the [Django administration panel](http://localhost:8080/admin). There you
     **Stop frame**. Frame on which video in task ends.
 
     **Frame Step**. Use this option to filter video frames.
-    For example, enter ``25`` to leave every twenty fifth frame in the video. Use this option on video files only.
+    For example, enter ``25`` to leave every twenty fifth frame in the video or every twenty fifth image. 
 
     **Chunk size**. Defines a number of frames to be packed in a chunk when send from client to server.
     Server defines automatically if empty. 
@@ -285,7 +285,7 @@ Go to the [Django administration panel](http://localhost:8080/admin). There you
     UX, only the first chunk of several frames will be loaded and you will be able
     to annotate first images. Other frames will be loaded in background.
 
-    ![](static/documentation/images/image007.jpg)
+    ![](static/documentation/images/image007_DETRAC.jpg)
 
 ### Models
 
@@ -359,19 +359,27 @@ The tool consists of:
 
 ### Basic navigation
 
-1.  Use arrows below to move on next/previous frame.
+1.  Use arrows below to move to the next/previous frame.
     Use the scroll bar slider to scroll through frames.
-    Almost every button is covered by a shortcut.
-    To get a hint about a shortcut, just put your mouse pointer over an UI element.
+    Almost every button has a shortcut.
+    To get a hint about a shortcut, just move your mouse pointer over an UI element.
 
     ![](static/documentation/images/image008.jpg)
 
-1.  An image can be scaled in/out using mouse's wheel. The image will be zoomed relatively your current cursor position.
-    Thus, if you point on an object, it will be under your mouse during zooming process.
+1.  To navigate the image, use the button on the controls sidebar. 
+    Another way an image can be moved/shifted is by holding the left mouse button inside 
+    an area without annotated objects. 
+    If the ``Mouse Wheel`` is pressed, then all annotated objects are ignored. Otherwise the
+    a highlighted bounding box will be moved instead of the image itself.
 
-1.  An image can be moved/shifted by holding left mouse button inside some area without annotated objects.
-    If ``Mouse Wheel`` is pressed, then all annotated objects are ignored.
-    Otherwise, a highlighted bounding box will be moved instead of the image itself.
+    ![](static/documentation/images/image136.jpg)
+
+1.  You can use the button on the sidebar controls to zoom on a region of interest. 
+    Use the button ``Fit the image`` to fit the image in the workspace.
+    You can also use the mouse wheel to scale the image 
+    (the image will be zoomed relatively to your current cursor position).
+
+    ![](static/documentation/images/image137.jpg)
 
 ### Types of shapes (basics)
 There are four shapes which you can annotate your images with: