minor #3769 Updated references to new Session() (scottwarren)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3769).

Discussion
----------

Updated references to new Session()

If you use new Session(), you get a 500 error, with the following message:

```log
Failed to start the session: already started by PHP.
```

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | I tested 2.4.2, but it most likely will be earlier version as well
| Fixed tickets | none

Commits
-------

fa86275 Updated warning for Sessions
4e06f0e fixed spaces and 72 char line limit
3624fec Updated grammer issues in caution section
b2ac5bc fixed typo
6e6b19f Fixed typo and removed extra space
df4ef14 Added extra new lines
275dd97 Added Caution note
a64f274 Removed use command
490efdc Updated references to new Session()

components/http_foundation/sessions.rst

@@ -12,6 +12,12 @@ object-oriented interface using a variety of session storage drivers.
 Sessions are used via the simple :class:`Symfony\\Component\\HttpFoundation\\Session\\Session`
 implementation of :class:`Symfony\\Component\\HttpFoundation\\Session\\SessionInterface` interface.
 
+.. caution::
+
+    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
+    http://symfony.com/doc/current/components/http_foundation/session_php_bridge.html
+
 Quick example::
 
     use Symfony\Component\HttpFoundation\Session\Session;