doc: Add '--manifest' example using 'specification->package+output'.

* doc/guix.texi (Invoking guix package): Add --manifest example using
'specification->package+output'.

doc/guix.texi

@@ -1685,6 +1685,20 @@ of packages:
        (list guile-2.0 "debug")))
 @end example
+@findex specification->package+output
+In this example we have to know which modules define the @code{emacs}
+and @code{guile-2.0} variables to provide the right
+@code{use-package-modules} line, which can be cumbersome.  We can
+instead provide regular package specifications and let
+@code{specification->package-output} look up the corresponding package
+objects, like this:
+
+@example
+(packages->manifest
+ (map (compose list specification->package+output)
+      '("emacs" "guile@@2.0" "guile@@2.0:debug")))
+@end example
+
 @item --roll-back
 @cindex rolling back
 @cindex undoing transactions