Added trans_default_domain tag to twig reference and some formatting

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.1+
| Fixed tickets | #1569

This PR is splitted in 3 commits:

 1. This adds the tag to the reference, this should be merged
 2. This reformatted the text about this tag in `book/translation` to match the common style, this is in for discussion
 3. This reformatted the reference links between the book article and the twig reference, I'm not sure of this