minor #17000 [Security] Document the getFirewallConfig() method (javiereguiluz)

This PR was squashed before being merged into the 6.2 branch.

Discussion
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[Security] Document the getFirewallConfig() method

Fixes #16855.

Commits
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3abdbc3 [Security] Document the getFirewallConfig() method

Author: javiereguiluz

security.rst

@@ -605,8 +605,42 @@ anything else within your firewall in the :ref:`access control
 
         $ composer require --dev symfony/profiler-pack
 
-Now that we understand our firewall, the next step is to create a way for your
-users to authenticate!
+Fetching the Firewall Configuration for a Request
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you need to get the configuration of the firewall that matched a given request,
+use the :class:`Symfony\\Bundle\\SecurityBundle\\Security\\Security` service::
+
+    // src/Service/ExampleService.php
+    // ...
+
+    use Symfony\Bundle\SecurityBundle\Security\Security;
+    use Symfony\Component\HttpFoundation\RequestStack;
+
+    class ExampleService
+    {
+        private Security $security;
+
+        public function __construct(Security $security, RequestStack $requestStack)
+        {
+            $this->requestStack = $requestStack;
+            // Avoid calling getFirewallConfig() in the constructor: auth may not
+            // be complete yet. Instead, store the entire Security object.
+            $this->security = $security;
+        }
+
+        public function someMethod()
+        {
+            $request = $this->requestStack->getCurrentRequest();
+            $firewallName = $this->security->getFirewallConfig($request)?->getName();
+
+            // ...
+        }
+    }
+
+.. versionadded:: 6.2
+
+    The ``getFirewallConfig()`` method was introduced in Symfony 6.2.
 
 .. _security-authenticators: