Add TODO about known return types in FFF

Author: Lipen

usvm-dataflow-ts/src/main/kotlin/org/usvm/dataflow/ts/infer/ForwardFlowFunctions.kt

@@ -452,6 +452,7 @@ class ForwardFlowFunctions(
         returnSite: EtsStmt,
     ): FlowFunction<ForwardTypeDomainFact> = FlowFunction { fact ->
         when (fact) {
+            // TODO: add known return type of the function call
             Zero -> listOf(Zero)
             is TypedVariable -> call(callStatement, fact)
         }