[Book] add note about services and the service container in the form cha...

...pter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

Services and the service container were not explained when reading the form chapter. Thus, I thought it is worth mentioning how to learn more about them if people get confused with all the new terms.