minor #9038 doc(testing/http_authentication.rst) (Sydney-o9, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #9038).

Discussion
----------

doc(testing/http_authentication.rst)

- [x] Added notes for people using Guard with the `PostAuthenticationGuardToken`

Commits
-------

1089de8 Simplify
a946faa doc(testing/http_authentication.rst);

Author: javiereguiluz

testing/http_authentication.rst

@@ -118,6 +118,8 @@ needs::
             // See https://symfony.com/doc/current/reference/configuration/security.html#firewall-context
             $firewallContext = 'secured_area';
 
+            // you may need to use a different token class depending on your application.
+            // for example, when using Guard authentication you must instantiate PostAuthenticationGuardToken
             $token = new UsernamePasswordToken('admin', null, $firewallName, array('ROLE_ADMIN'));
             $session->set('_security_'.$firewallContext, serialize($token));
             $session->save();