Add clarifying comments

Author: Benjamin Lichtman

src/services/codefixes/convertToAsyncFunction.ts

@@ -393,9 +393,10 @@ namespace ts.codefix {
         const shouldReturn = transformer.setOfExpressionsToReturn.get(getNodeId(parent).toString());
         switch (func.kind) {
             case SyntaxKind.NullKeyword:
-                // do not produce a transformed statement for a null or undefined argument
+                // do not produce a transformed statement for a null argument
                 break;
             case SyntaxKind.Identifier:
+                // identifier includes undefined
                 if (!hasArgName) break;
 
                 const synthCall = createCall(getSynthesizedDeepClone(func) as Identifier, /*typeArguments*/ undefined, [argName.identifier]);

src/services/suggestionDiagnostics.ts

@@ -196,7 +196,7 @@ namespace ts {
     function isFixablePromiseArgument(arg: Expression): boolean {
         switch (arg.kind) {
             case SyntaxKind.NullKeyword:
-            case SyntaxKind.Identifier:
+            case SyntaxKind.Identifier: // identifier includes undefined
             case SyntaxKind.FunctionDeclaration:
             case SyntaxKind.FunctionExpression:
             case SyntaxKind.ArrowFunction: