Use JSDoc notation for property default and optional-ness

Per Lyza's suggestion and matches the examples at
https://www.typescriptlang.org/docs/handbook/type-checking-javascript-files.html

src/sidebar/services/toast-messenger.js

@@ -14,8 +14,7 @@ const MESSAGE_DISMISS_DELAY = 500;
  * Additional control over the display of a particular message.
  *
  * @typedef {Object} MessageOptions
- * @prop {boolean} autoDismiss - Whether the toast message automatically disappears.
- *   Defaults to true.
+ * @prop {boolean} [autoDismiss=true] - Whether the toast message automatically disappears.
  */
 
 // @ngInject