guix: ui: Improve missing output descriptions.

The current reference to Intel's Appendix H appears too obscure and confusing.

* guix/ui.scm (package->recutils)[output->recutils]: Replace “see Appendix H”
with “[description missing]”.

guix/ui.scm

@@ -1531,7 +1531,7 @@ that may return a colorized version of its argument."
          (_ #f))
        (package-properties package))
       (assoc-ref default-synopses output)
-      (G_ "see Appendix H"))))
+      (G_ "[description missing]"))))
 
   (define (package-outputs/out-last package)
     ((compose append partition)