docs: API level documentation for Creator Functions

[jtcrowson]

I'd like to help NgRx achieve better API level documentation, similar to angular.io/api.

I'd like to submit a PR to create JSDocs for createAction, createReducer, and createEffect.

This would be a good place to put implementation details, like for createReducer, the fact that only 10 actions can be provided per on, and duplicated action type handling (#1956).

If accepted, I would be willing to submit a PR for this feature

[x] Yes (Assistance is provided if you need help submitting a pull request)
[ ] No