Add comment to new code in hasExcessProperties

Author: sandersn

src/compiler/checker.ts

@@ -8955,6 +8955,7 @@ namespace ts {
                                     reportError(Diagnostics.Property_0_does_not_exist_on_type_1, symbolToString(prop), typeToString(target));
                                 }
                                 else {
+                                    // use the property's value declaration if the property is assigned inside the literal itself
                                     const objectLiteralDeclaration = source.symbol && firstOrUndefined(source.symbol.declarations);
                                     if (prop.valueDeclaration && findAncestor(prop.valueDeclaration, d => d === objectLiteralDeclaration)) {
                                         errorNode = prop.valueDeclaration;