docs for revised system error message completed

(manually adapted)

(cherry picked from commit 6768136144)

collects/mzlib/process.rkt

@@ -6,7 +6,10 @@
          system
          system*
          system/exit-code
-         system*/exit-code)
+         system*/exit-code
+
+	 string-no-nuls?
+	 bytes-no-nuls?)
 
 (require "private/streams.rkt")
 
@@ -82,9 +85,6 @@
          s)))])
   args)
 
-;; MF: fxing a somewhat awkward looking error message. Comment for Matthew in case he wants to improve on it. 
-(provide string-no-nuls? bytes-no-nuls?)
-
 (define (check-command who str)
   (unless (or (string-no-nuls? str)
               (bytes-no-nuls? str))

collects/scribblings/reference/subprocess.scrbl

@@ -482,3 +482,17 @@ returned list is @racket[#f].}
 Like @racket[process*], but with the port handling of
 @racket[process/ports].}
 
+@; ----------------------------------------------------------------------
+
+@;section{Contract Auxiliaries}
+
+@;note-lib[racket/system]
+
+The contracts of @racket[system] and related functions may signal a
+contract error with references to the following functions. 
+
+@defproc[(string-no-nuls? [x any/c]) boolean?]{
+Ensures that @racket[x] is a string and does not contain @racket["\0"].}
+
+@defproc[(bytes-no-nuls? [x any/c]) boolean?]{
+Ensures that @racket[x] is a byte-string and does not contain @racket["\0"].}

doc/release-notes/racket/HISTORY.txt

@@ -77,6 +77,7 @@ racket/sandbox: added sandbox-propagate-exceptions; added
 racket/cmdline: add #:ps for command-line
 slideshow/start: run a `slideshow' or `main' submodule, if any
 scribble/eval: added eval:result and eval:results
+racket/system: improved error messages by replacing memv expressions with string-no-nuls? and bytes-no-nuls?
 
 Version 5.2.1, January 2012
 Changed I/O scheduling to use epoll()/kqueue() when available