doc: Give a complete example of adding setuid programs.

* doc/guix.texi (Setuid Programs): Give a complete example of how to use
SETUID-PROGRAM.

doc/guix.texi

@@ -34005,6 +34005,16 @@ designated like this:
   (program (file-append shadow "/bin/passwd")))
 @end example
 
+And then it can be added to your operating system declaration by
+appending it to @code{%setuid-programs} like this:
+
+@example
+(setuid-programs
+  (append (list (setuid-program
+                  (program (file-append swaylock "/bin/swaylock"))))
+                %setuid-programs))
+@end example
+
 @deftp {Data Type} setuid-program
 This data type represents a program with a setuid or setgid bit set.