Tweaks after proofreading the 2.6 OptionsResolver stuff

Hi guys!

This is a proofread after merging #4159. There were enough changes there that the easiest way to handle things was to merge, re-read entirely, then open a PR with small tweaks. But largely, I am very happy with how the document turned out :).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6+
| Fixed tickets | --

Thanks!
No. While we should follow the CS of Symfony as much as possible, I don't think we should do it here. Not aligning items will significantly reduce the readability of the code. In documentation, the code is purely meant for presentation. In the core code, code also have other tasks than presentation.