Clarify Datalog/Racket interaction re Matthias

original commit: 2846764a93d3317b0934c61d078f54baf3a18599

pkgs/datalog/scribblings/racket.scrbl

@@ -61,6 +61,8 @@ The Datalog database can be directly used by Racket programs through this API.
 
           (datalog family
                    (? (add1 1 :- X)))
+          (datalog family
+                   (? (add1 X :- 2)))
           (datalog family
                    (? (#,(λ (x) (+ x 1)) 1 :- X)))]
 
@@ -107,7 +109,13 @@ for a variable symbol, or @RACKET[#,expr] where @racket[expr]
 evaluates to a constant datum. Bound identifiers in terms are treated
 as the datum they are bound to.
 
-External queries invalidate Datalog's guaranteed termination. For example, this program does not terminate:
+External queries fail if any logic variable is not fully resolved to a
+datum on the Datalog side. In other words, unbound logic variables
+never flow to Racket.
+
+External queries invalidate Datalog's guaranteed termination. For
+example, this program does not terminate:
+
 @racketblock[
  (datalog (make-theory)
           (! (:- (loop X)

pkgs/datalog/tests/racket.rkt

@@ -76,6 +76,11 @@
  =>
  (list (hasheq 'X 2))
 
+ (datalog parent
+          (? (add1 X :- 2)))
+ =>
+ (list)
+
  (datalog parent
           (? (#,(λ (x) (+ x 1)) 1 :- X)))
  =>