Comment and clarify top-level evaluation & barriers

collects/scribblings/inside/eval.scrbl

@@ -24,7 +24,8 @@ followed by @cppi{scheme_eval_compiled}.
 
 The functions @cpp{scheme_eval}, @cpp{scheme_apply}, etc., are
 @defterm{top-level evaluation functions}. Continuation invocations are
-confined to jumps within a top-level evaluation.
+confined to jumps within a top-level evaluation (i.e., a continuation
+barrier is installed by these functions).
 
 The functions @cppi{_scheme_eval_compiled}, @cppi{_scheme_apply},
 etc. (with a leading underscore) provide the same functionality

src/racket/src/fun.c

@@ -5632,6 +5632,8 @@ call_with_continuation_barrier (int argc, Scheme_Object *argv[])
 {
   scheme_check_proc_arity("call-with-continuation-barrier", 0, 0, argc, argv);
 
+  /* scheme_apply_multi() is a top-level evaluation function and will
+     thus install a continuation barrier */
   return scheme_apply_multi(argv[0], 0, NULL);
 }