Clarify a statement in UB test

src/test/ui/consts/const-eval/double_check2.rs

@@ -1,7 +1,9 @@
 // check-pass
 
 // This test exhibits undefined behavior, but it is impossible to prevent generally during
-// const eval, even if possible to prevent in the cases here. The reason it's impossible in general
+// const eval, even if it could be prevented in the cases here if we added expensive and
+// complex checks in rustc.
+// The reason it's impossible in general
 // is that we run into query cycles even *without* UB, just because we're checking for UB.
 // We do not detect it if you create references to statics
 // in ways that are UB.