Is the comment about the full-stack framework adding anything here? I think it might be confusing what we're saying. As I understand it, the important message here is "make sure the PHP session hasn't already been started in some other way before using the `Session` object, else you'll get an error". Can we remove the full-stack framework comment to add clarity?

And if I'm understanding the message correctly, I think we can say it in one really quick sentence:

>
Make sure your PHP session isn't already started before using the `Session` class. If you have a legacy session system that starts your session, see [link to http://symfony.com/doc/current/components/http_foundation/session_php_bridge.html]