Documented all form variables

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | #2372

This PR adds all missing global form variables to the twig reference.

Besides that, it also introduces a new section for form types: Form
Variables. This explains what kind of variables are available for that
particular field. This is usefull if you want to create a custom theme.

At last, it also adds the missing docs for the `selectedchoice` Twig
test.