[Book] add note about services and the service container in the form chapter
minor #3766 [Book] add note about services and the service container in the form cha... (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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4afd8a4 [Book] add note about services and the service container in the form chapter