reodered and tweaked the book section

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | https://github.com/symfony-cmf/symfony-cmf-docs/commit/5d2a3b4b5f85c1aa2285322ab83c0f8df5ea3606#commitcomment-4042808, https://github.com/symfony-cmf/symfony-cmf-docs/commit/5bb40f64ff1333cfbf49067ba2e7c79f5e627fe3#commitcomment-4042834
| Fixed tickets | -

@WouterJ This addresses some of the concerns I raised.