Add a note that Symfony's recipe enables bundle for `prod`.

[MarcinOrlowski]

Problem Statement

Official integration docs for Symfony: https://docs.sentry.io/platforms/php/guides/symfony/ shows steps to follow to record first test error. But it lacks vital information, that when Symfony package is installed its recipe configures the bundle to be available in prod environment only. There's high chance that one will try to test it locally first before deployment and because s/he is usually on dev environment in that case, nothing will be really recorded giving false impression something is not working. Adding a note about that nuance would most likely be useful for new users (and yes, I stepped on this).

Solution Brainstorm

Update the docs.