removed _contract_ language from _invariant-..._ as much as possible

added a hint as to why the error message uses the inappropriate contract language

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

@@ -1509,24 +1509,24 @@ The @racket[define-struct/contract] form only allows a subset of the
 (make-salmon #f 'pacific)
 ]}
 
-@defform[(invariant-contract contract-expr expr)]{
-  Establishes an invariant of @racket[expr], determined by @racket[contract-expr].
+@defform[(invariant-assertion invariant-expr expr)]{
+  Establishes an invariant of @racket[expr], determined by @racket[invariant-expr].
                    
-  Unlike other ways to attach contracts to values, an
-  @racket[invariant-contract] does not establish a boundary
-  between two parties. Instead, it simply puts the contract
-  on the value, treating the module containing the 
-  @racket[invariant-contract] expression as the party to be blamed
-  for any violations of the contract.
+  Unlike the specification of a contract, an
+  @racket[invariant-assertion] does not establish a boundary
+  between two parties. Instead, it simply attaches a logical assertion 
+  to the value. Because the form uses contract machinery to check the
+  assertion, the surround module is treated as the party to be blamed
+  for any violations of the assertion. 
   
-  This means, for example, that the contract is checked on
+  This means, for example, that the assertion is checked on
   recursive calls, when an invariant is used on the right-hand
-  side of a definition.
+  side of a definition:
                               
   @examples[#:eval 
             furlongs->feet-eval
             (define furlongss->feets
-              (invariant-contract
+              (invariant-assertion
                (-> (listof real?) (listof real?))
                (λ (l)
                  (cond

racket/collects/racket/contract/private/base.rkt

@@ -3,7 +3,7 @@
 (provide contract
          (rename-out [-recursive-contract recursive-contract])
          current-contract-region
-         invariant-contract)
+         invariant-assertion)
 
 (require (for-syntax racket/base syntax/name syntax/srcloc)
          racket/stxparam
@@ -89,7 +89,7 @@
           (procedure-rename new-val vs-name)])]
       [else new-val])))
 
-(define-syntax (invariant-contract stx)
+(define-syntax (invariant-assertion stx)
   (syntax-case stx ()
     [(_ ctc e)
      (quasisyntax/loc stx

racket/collects/racket/contract/region.rkt

@@ -4,7 +4,7 @@
          define/contract
          with-contract
          current-contract-region
-         invariant-contract)
+         invariant-assertion)
 
 (require (for-syntax racket/base
                      racket/struct-info