doc: Add 'cups-configuration' example with 'hplip'.

* doc/guix.texi (Printing Services): Add example with 'hplip'.

doc/guix.texi

@@ -9288,6 +9288,7 @@ makes the good ol' XlockMore usable.
 @node Printing Services
 @subsubsection Printing Services
 
+@cindex printer support with CUPS
 The @code{(gnu services cups)} module provides a Guix service definition
 for the CUPS printing service.  To add printer support to a GuixSD
 system, add a @code{cups-service} to the operating system definition:
@@ -9308,13 +9309,17 @@ as GNOME's printer configuration services.  By default, configuring a
 CUPS service will generate a self-signed certificate if needed, for
 secure connections to the print server.
 
-One way you might want to customize CUPS is to enable or disable the web
-interface.  You can do that directly, like this:
+Suppose you want to enable the Web interface of CUPS and also add
+support for HP printers @i{via} the @code{hplip} package.  You can do
+that directly, like this (you need to use the @code{(gnu packages cups)}
+module):
 
 @example
 (service cups-service-type
          (cups-configuration
-           (web-interface? #f)))
+           (web-interface? #t)
+           (extensions
+             (list cups-filters hplip))))
 @end example
 
 The available configuration parameters follow.  Each parameter