[Process] Document that environment variables are inherited to child processes

[mhujer]

As discussed in symfony/symfony#29722, environment variables are inherited to a child process unless specified otherwise.

It may lead to confusion when trying to run command in other environment with --env=test (even in different app). I suggest that something like this is added to https://symfony.com/doc/current/components/process.html (or maybe a whole section with a more detailed explanation):

Caution: Environment variables are always inherited to child process unless you pass ['APP_ENV' => false, 'SYMFONY_DOTENV_VARS' => false] when creating new instance of Symfony\Component\Process\Process.

[mhujer]

@javiereguiluz thank you! 👍