Issues with the Twig documentation, missing/unavailable functions

[sandoba]

I've noticed a few issues with the documentation, especially in regards to additional functionality provided by Symfony:

1. the tag "transchoice" mentioned at https://twig.symfony.com/doc/3.x/ is not part of the Symfony documentation, e.g. not mentioned in https://symfony.com/doc/current/reference/twig_reference.html or in any other page of the documentation

2. various functions and filters from https://symfony.com/doc/current/reference/twig_reference.html aren't listed at https://twig.symfony.com/doc/3.x/, e.g. "impersonation_exit_path", "impersonation_exit_url", "sanitize_html" and "serialize"

3. https://symfony.com/doc/current/form/form_customization.html mentions an additional function "form_parent" which is also not listed at https://twig.symfony.com/doc/3.x/

4. the function "form" links to https://symfony.com/doc/current/reference/twig_reference.html#form with an incorrect anchor, probably best to link directly to https://symfony.com/doc/current/form/form_customization.html as well

5. the link "Logic" at https://twig.symfony.com/doc/3.x/ refers to the wrong paragraph "Math"