- Add JSDoc for the `actionTypes` function. This uses an advanced
  TS feature called "mapped types" [1] to specify that the result is an
  object with the same keys as the input object, but where the values
  are strings rather than functions.

  Going forwards there are likely better ways to replace `actionTypes`.
  The original purpose was to make sure that the `type` field of each action
  object matches up with a key of the `updates` object.

- Fix various existing JSDoc errors
- Add imports for the existing `Group` and `Annotation` references
- Add missing `group` property to `Annotation` type

[1] https://www.typescriptlang.org/docs/handbook/advanced-types.html#mapped-types
    (See in particular the `in keyof T` mentions).