JS: Explain false positive in test case

javascript/ql/test/library-tests/TripleDot/useuse.js

@@ -163,6 +163,8 @@ function t9() { // same as t8 but with a SanitizerGuard that isn't just a variab
         }
 
         if (typeof obj === "undefined" || typeof obj === "undefined") {
+            // The shared SSA library expects short-circuiting operators be pre-order in the CFG,
+            // but in JS they are post-order (as per evaluation order).
             sink(obj.field); // $ SPURIOUS: hasTaintFlow=t9.1
         } else {
             sink(obj.field); // $ hasTaintFlow=t9.1