Add #:tag to the docs for define-cpointer-type

pkgs/racket-doc/scribblings/foreign/cpointer.scrbl

@@ -57,9 +57,14 @@ are not tagged.}
 
 
 @defform*[[(define-cpointer-type _id)
+           (define-cpointer-type _id #:tag tag-id)
            (define-cpointer-type _id ptr-type-expr)
+           (define-cpointer-type _id ptr-type-expr #:tag tag-id)
            (define-cpointer-type _id ptr-type-expr 
-                                 racket-to-c-expr c-to-racket-expr)]]{
+                                 racket-to-c-expr c-to-racket-expr)
+           (define-cpointer-type _id ptr-type-expr 
+                                 racket-to-c-expr c-to-racket-expr
+                                 #:tag tag-id)]]{
 
 A macro version of @racket[_cpointer] and @racket[_cpointer/null],
 using the defined name for a tag symbol, and defining a predicate
@@ -72,7 +77,8 @@ In addition to defining @racket[_id] to a type generated by
 type produced by @racket[_cpointer/null] type. Finally,
 @racketvarfont{id}@racketidfont{?}  is defined as a predicate, and
 @racketvarfont{id}@racketidfont{-tag} is defined as an accessor to
-obtain a tag. The tag is the symbol form of @racketvarfont{id}.}
+obtain a tag. If provided, the tag is @racket[tag-id], otherwise
+it is the symbol form of @racketvarfont{id}.}
 
 @defproc[(cpointer-predicate-procedure? [v any/c]) boolean?]{Returns
  @racket[#t] if @racket[v] is a predicate procedure generated by