C++: Reinstate accidentally-deleted comment

Author: calumgrant

cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql

@@ -57,5 +57,5 @@ where
   not declarationHasSideEffects(v) and
   not exists(AsmStmt s | f = s.getEnclosingFunction()) and
   not v.getAnAttribute().getName() = "unused" and
-  not f.hasErrors()
+  not f.hasErrors() // Unextracted expressions may use `v`
 select v, "Variable " + v.getName() + " is not used."