Book/Form : Adding a new section about defining forms as service

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed ticket  | -
Clarifying form / form type
Fixing typo
Following @WouterJ recommandations
Merge pull request #2667 from gregquat/patch-15

Book/Form : Adding a new section about defining forms as service