Update documentation for checkbox-group

Author: tojoqk

web-server-doc/web-server/scribblings/formlets.scrbl

@@ -381,15 +381,17 @@ element of the sequence.
 @defproc[(checkbox-group [l sequence?]
                       [#:attributes attrs (any/c . -> . (listof (list/c symbol? string?))) (λ (x) empty)]
                       [#:checked? checked? (any/c . -> . boolean?) (λ (x) #f)]
-                      [#:display display (any/c . -> . xexpr/c) (λ (x) x)])
+                      [#:display display (any/c . -> . xexpr/c) (λ (x) x)]
+                      [#:wrap wrap (any/c any/c . -> . xexpr/c) (λ (x y) (list x y))])
         (formlet/c (listof any/c))]{
 
   This @tech{formlet} renders using a sequence of INPUT elements of
 CHECKBOX type where each element gets its attributes from
 @racket[attrs] that share a single NAME. An element is checked if
-@racket[checked?]  returns @racket[#t]. Elements are followed by the
-results of @racket[display]. The result of processing this formlet is
-a list of elements of the sequence.
+@racket[checked?]  returns @racket[#t]. Elements are combined with the
+results of @racket[display] into an X-expression specified in
+@racket[wrap]. The result of processing this formlet is a list of
+elements of the sequence.
 }
 
 @defproc[(submit [value (or/c bytes? string?)]