Document contract-continuation-mark-key.

pkgs/contract-profile/main.rkt

@@ -2,7 +2,6 @@
 
 (require racket/list unstable/list racket/match racket/set racket/format
          racket/contract
-         (only-in racket/contract/private/guts contract-continuation-mark-key)
          profile/sampler profile/utils profile/analyzer
          "dot.rkt" "utils.rkt" "boundary-view.rkt")
 

pkgs/racket-pkgs/racket-doc/scribblings/reference/contracts.scrbl

@@ -2306,6 +2306,12 @@ search tree invariant. Removing the @racket[-opt/c] also
 makes a binary search tree contract, but one that is
 (approximately) 20 times slower.}
 
+@defthing[contract-continuation-mark-key continuation-mark-key?]{
+Key used by continuation marks that are present during contract checking.
+The value of these marks are the blame objects that correspond to the contract
+currently being checked.
+}
+
 @section{@racketmodname[racket/contract/base]}
 
 @defmodule[racket/contract/base]

racket/lib/collects/racket/contract/base.rkt

@@ -48,4 +48,5 @@
  ;; from private/guts.rkt
  has-contract?
  value-contract
+ contract-continuation-mark-key
  )