Add video_thumbnail_second docs

docs/generating_the_url_advanced.md

@@ -411,6 +411,15 @@ gifo:%gif_optimize_frames:%gif_optimize_transparency
 
 Allows redefining GIF saving options. All arguments have the same meaning as [Advanced GIF compression](configuration.md#advanced-gif-compression) configs. All arguments are optional and can be omitted.
 
+#### Video thumnail second <img class="pro-badge" src="assets/pro.svg" alt="pro" />
+
+```
+video_thumbnail_second:%second
+vts:%second
+```
+
+Allows redefining `IMGPROXY_VIDEO_THUMBNAIL_SECOND` config.
+
 #### Preset
 
 ```