Symfony version(s) affected: 4.1

Description
Following the documentation on a fresh installed Symfony 4.1.
https://symfony.com/doc/current/configuration/external_parameters.html#environment-variable-processors

Copying and pasting these configuration in config/packages/framework.yaml

Getting an exception:

In BaseNode.php line 529:
Invalid type for path "framework.trusted_hosts.0". Expected one of "bool", "int", "float", "string", but got "array".

My environment does not contain a TRUSTED_PROXIES variable.
It looks like a bug with a JSON environment variables passed to configuration. If it's by design than it's a documentation bug.

Maybe related to #28137 and #23888