minor #9733 Documentation about authentication for multiple firewalls under same context (Valantis Koutsoumpos, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #9733).

Discussion
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Documentation about authentication for multiple firewalls under same context

Added documentation about authentication for multiple firewalls under the same firewall context

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Commits
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eae4a3c Reword
d430972 Added documentation about authentication for multiple firewalls under the same firewall context

Author: javiereguiluz

testing/http_authentication.rst

@@ -113,10 +113,12 @@ needs::
         {
             $session = $this->client->getContainer()->get('session');
 
-            // the firewall context defaults to the firewall name
+            $firewallName = 'secure_area';
+            // if you don't define multiple connected firewalls, the context defaults to the firewall name
+            // See https://symfony.com/doc/current/reference/configuration/security.html#firewall-context
             $firewallContext = 'secured_area';
 
-            $token = new UsernamePasswordToken('admin', null, $firewallContext, array('ROLE_ADMIN'));
+            $token = new UsernamePasswordToken('admin', null, $firewallName, array('ROLE_ADMIN'));
             $session->set('_security_'.$firewallContext, serialize($token));
             $session->save();