Document the escape Twig filter

[fritzmg]

@ausi @m-vo I would need some help here. I want to document the exact differences of the escape strategy that Contao introduces - and also document examples regarding ChunkedText from insert tags.

[ausi]

The escape differences are described here: https://docs.contao.org/dev/framework/templates/architecture/#double-encoding-prevention

A ChunkedText object contains strings that need to be escaped and other strings that are kept raw. So {{ text|insert_tag_raw|escape('html') }} only escapes the parts that do not come from the replaced insert tags. However this is not directly related to the Contao escaper, this should work in all Twig templates (not only @Contao and @Contao_*). See https://github.com/contao/contao/blob/4f485fe5410d7ec1e7c9efc9ee5972844d094aec/core-bundle/src/Twig/Extension/ContaoExtension.php#L282-L292