Update docs to cover literal type

Author: Hyunje Jun

docs/pages/guide/typescript.md

@@ -47,6 +47,10 @@ const message: Line.TemplateMessage = {
 The object above will be type-checked to have the type of
 `Line.TemplateMessage`, and thus ensured not to miss any required field.
 
+Also, [literal type](https://www.typescriptlang.org/docs/handbook/advanced-types.html)
+is used for `type` fields, which means the compiler will complain if a wrong
+type string is used, and also inference the type of objects by its `type` field.
+
 ## How to use
 
 The library is built to just-work with TypeScript too, so import the library and