[gnome-user-docs] Add description of scrollbar middle-clicking

[David King <davidk src gnome org>]

commit aa3f098ff0874b9bb77e3e38a076899efc06eaa6
Author: David King <amigadave amigadave com>
Date:   Fri Jul 19 19:30:38 2019 +0100

    Add description of scrollbar middle-clicking
    
    Fixes: https://gitlab.gnome.org/GNOME/gnome-user-docs/issues/20

 gnome-help/C/mouse-middleclick.page | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gnome-help/C/mouse-middleclick.page b/gnome-help/C/mouse-middleclick.page
index b3aed9d2..bbbb766a 100644
--- a/gnome-help/C/mouse-middleclick.page
+++ b/gnome-help/C/mouse-middleclick.page
@@ -8,6 +8,7 @@
 
     <revision pkgversion="3.8" date="2013-03-13" status="candidate"/>
     <revision pkgversion="3.18" date="2015-09-28" status="final"/>
+    <revision pkgversion="3.33.4" date="2019-07-19" status="candidate"/>
 
     <credit type="author">
       <name>Tiffany Antopolski</name>
@@ -49,6 +50,10 @@ touchpad settings for this to work.</p>
 <p>Many applications use middle-click for advanced click shortcuts.</p>
 
 <list>
+  <item><p>In applications with scrollbars, left-clicking in the empty space of
+  the bar moves the scroll position directly to that place. Middle-clicking
+  moves up to a single page towards that location.</p></item>
+
   <item><p>In the <gui>Activities</gui> overview, you can quickly open a new
   window for an application with middle-click. Simply middle-click on the
   application’s icon, either in the dash on the left, or in the applications