[gnome-user-docs] Ubuntu note about GNOME Classic prerequisite

[Shaun McCance <shaunm src gnome org>]

commit 323bdf23b497d485c8a3cb8af3c5ddf583d3e371
Author: Gunnar Hjalmarsson <gunnarhj ubuntu com>
Date:   Sat Dec 11 00:54:23 2021 +0100

    Ubuntu note about GNOME Classic prerequisite
    
    Closes: #142

 gnome-help/C/gnome-classic.page | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/gnome-help/C/gnome-classic.page b/gnome-help/C/gnome-classic.page
index 7f3d6b20..fe6dc913 100644
--- a/gnome-help/C/gnome-classic.page
+++ b/gnome-help/C/gnome-classic.page
@@ -73,12 +73,19 @@
 <section id="gnome-classic-switch">
 <title>Switch to and from GNOME Classic</title>
 
-<note if:test="!platform:gnome-classic" style="important">
+<note if:test="!platform:gnome-classic !platform:ubuntu" style="important">
 <p>GNOME Classic is only available on systems with certain GNOME Shell extensions
  installed. Some Linux distributions may not have these extensions available or
  installed by default.</p>
 </note>
 
+<note if:test="!platform:gnome-classic platform:ubuntu" style="important">
+<p its:locNote="Translators: Ubuntu only string">You need to have the
+ <sys>gnome-shell-extensions</sys> package installed to make GNOME Classic available.</p>
+<p its:locNote="Translators: Ubuntu only string"><link style="button"
+ action="install:gnome-shell-extensions">Install <sys>gnome-shell-extensions</sys></link></p>
+</note>
+
   <steps>
     <title>To switch from <em>GNOME</em> to <em>GNOME Classic</em>:</title>
     <item>