racket/sandbox: change syntax-object wrappers generated for locations

Generate wrappers consistent with a reader-level absence of `.'s,
instead of wrapping every pair. This change fixes the `syntax-e'
example in the Guide's description of syntax objects.

collects/racket/sandbox.rkt

@@ -503,6 +503,12 @@
 
 (define (add-location x loc)
   (cond [(null? x) null]
+        [(list? x)
+         ;; For a list, generate a syntax-object wrapper consistent with 
+         ;; an absence of `.'s in the reader's input:
+         (make-orig (for/list ([i (in-list x)])
+                      (add-location i loc))
+                    loc)]
         [(pair? x) (make-orig (cons (add-location (car x) loc) 
                                     (add-location (cdr x) loc))
                               loc)]

collects/tests/scribble/docs/stxobj.scrbl

@@ -0,0 +1,6 @@
+#lang scribble/manual
+@(require scribble/eval)
+
+@interaction[
+(syntax-e #'(+ 1 2))
+]

collects/tests/scribble/docs/stxobj.txt

@@ -0,0 +1,2 @@
+  > (syntax-e #'(+ 1 2))                            
+  '(#<syntax:1:0 +> #<syntax:1:0 1> #<syntax:1:0 2>)